Ontogeny and phylogeny
All animals must reproduce, passing copies of their genes into separate new bodies in future generations. These genetic copies may be genetically identical, produced by asexual processes, or genetically distinct, produced by sexual processes. In sexual processes, which are by far the more common among animals, the initial result of reproduction is a single cell, known as a zygote, containing the new and unique set of genes. Yet, by definition animals are multicellular, and generally consist of hundreds, thousands, or millions of cells. Even more important is the fact that the assemblage of cells that we recognize as a given animal species must be organized into a specific pattern. This pattern, when viewed as a whole, defines the morphology of the animal. The morphology, in turn, underlies a complex functional organization of the animal in which the cells are grouped into tissues (such as epidermis), organs (such as kidneys), and organ systems (such as digestive systems). Each separate group of cells within an animal's body performs a specific function in what is called the division of labor among body cells. The processes through which the single zygote becomes the complex multicellular adult animal, with many tissues, organs, and systems in their proper places, all functioning in a coordinated manner, are referred to as the animal's ontogeny.
Clearly, the ontogeny of an animal is critical to determining what kind of creature the animal is and what it can become. Thus, it should be no surprise that specific ontogenetic patterns tend to be characteristic of particular groups of animals. For example, the leg of a crab and the leg of a mouse are very different structures; the crab has an epidermal ex-oskeleton around muscles, whereas the mouse has muscles and epidermis around an internal bony skeleton. The crab's structural characteristics define it as a member of the phylum Arthropoda; the mouse's structural characteristics define it as a member of the phylum Chordata. Since structure must come about through ontogeny, it stands to reason that each morphologically distinct phylum of animals must also have a distinctive ontogenetic pattern. So, taking it one step further, we can reason that an animal's ontogeny (the developmental history of the individual), correlates with its phylogeny (the evolutionary history of the phylum).
Some nineteenth-century naturalists and biologists were so struck by this relationship that they argued that an ani mal's entire evolutionary history was repeated during the course of its embryonic development as an individual. Later studies have shown that this exact repetition does not occur. However, the pattern of ontogeny is so important to an animal's formation that a basic correlation between ontogeny and phylogeny does exist. For this reason, comparative zoologists have long regarded patterns of embryonic development as being crucial to the understanding of where each group of animals fits into the larger phylogenetic scheme.
Given the long-standing recognition that embryonic developmental patterns reflect evolutionary relationships, it comes as no surprise that the two major branches of the animal kingdom are defined by differences in specific embryonic attributes. Early metazoans, such as sponges (phylum Porifera) and jellyfishes (phylum Cnidaria), have rather simple bodies that exhibit a rather high degree of developmental plasticity. However, with the advent of flatworms (phylum Platyhelminthes), we see greater complexity, and generally less plasticity. In flatworms and all higher animals, the body forms early into a three-layered embryo, and thus is said to be triploblastic. Each of the three layers is known as a germ layer, because it will form into all the organs of that body layer. The outer ectoderm layer will develop into external structures such as the epidermis, skin, exoskeleton, nervous system, and sensory structures. The middle mesoderm layer becomes internal organs such as kidneys, reproductive systems, circulatory systems, and muscles. The inner endoderm layer will develop into the gut cavity and its derivatives, such as the stomach, intestine, and liver.
Above the level of flatworms, all higher animals possess an additional mesodermal feature—a membrane-lined body cavity, or coelom. This feature is so important that these higher animals, which constitute more than 85% of animal species, are known collectively as coelomates. But the coelom forms in two very different ways, each of which corresponds generally with two very different sequences of basic embryonic events. Thus, the higher animals fall into two great branches, the Deuterostomia and the Protostomia, each defined by a unique set of embryonic characteristics. The variations relate to (1) the pattern of cleavage; (2) the fate of the embryonic
blastopore formed during gastrulation; and (3) the method of coelom formation. The relevant characteristics among the protostomes are described in the following sections.
As higher animals, all sexually reproducing protostomes form gametes, generally in the form of sperm (in male systems) and oocytes (in female systems). (Oocytes are sometimes ambiguously called "eggs.") Most protostomes are gonocho-ristic, meaning that they have separate male and female individuals. However, hermaphroditism, or the formation of male and female gametes in the same individual, is very common in many protostome phyla, reaching high levels in groups such as the leeches and earthworms (phylum Annelida), and the pulmonate snails (phylum Mollusca). Among hermaphrodites, sperm and oocytes can be produced by the same gonad, or by separate male and female gonads, depending on the species. In any case, the basic processes of gamete formation, or gametogenesis, are fundamentally similar in all phyla. In all cases, it begins with reduction of the chromosome number from paired sets to single sets, so that subsequent joining of pairs from the male and female results in restoration of paired sets. After the reductional division, each gamete must take on structural and functional characteristics that enable it to engage in pairing with the gamete of the opposite sex.
Spermatogenesis, the formation of sperm, thus begins with reductional division, then proceeds to development of a generally motile and diminutive cell, capable of positioning itself in physical contact with the oocyte. Although it is technically true that most sperm can swim, in some species sperm can crawl, slither, or glide. In each of these cases, however, it is important to note that sperm cannot travel great distances; the various propulsion devices, therefore, are more important for small-scale positioning than for actually seeking out the oocyte. This is especially true of the many marine proto-stomes that spawn their naked gametes directly into the sea-water. The formation of an individual sperm generally involves extreme condensation of the chromosomes, and elaboration of motility devices such as flagella, oocyte-encounter and oocyte-manipulation devices, and energy stores.
An important aspect of spermatogenesis in most species is the close synchronization of sperm development and release. The primary basis for synchronization is the maintenance of close physical contact between the spermatogenic cells throughout their development. In virtually all cases, this contact involves the actual sharing of a common cell membrane and cytoplasm among large clusters of cells. These clusters are known as spermatogenic morulae. In annelid worms, velvet worms (phylum Onychophora), and some other protostome groups, these morulae have the appearance of balls of sperm, all within a large saclike gonad or the body cavity, with the heads pointed inward and the tails pointing outward. In shrimp, lobsters, and other crustaceans, as well as insects (phylum Arthropoda), the morulae occupy individual chambers in the gonad. In snails, clams, and their relatives (phylum Mol-lusca), the morulae often occur in concentric rings, with the less-developed cells in the outer rings, near the gonad wall, and the more-developed cells in the central rings, near to the ducts leading to the outside.
Oogenesis, the formation of oocytes, also begins with a re-ductional division of the chromosomes, but then proceeds to the formation of a generally large, spherical, nonmotile cell. The oocyte does not generally contribute to preliminary positioning with the sperm, but it does play a vital role in bringing the two gametes to a point of fusing to form a single composite cell, the fertilized zygote. In fact, contrary to popular belief, it is more correct to say that the oocyte fertilizes the sperm, rather than that the sperm fertilizes the oocyte. In reality, both gametes make vital contributions to this union, but it is clearly the oocyte that is responsible for most of what happens after that. During oogenesis, the oocyte is equipped with special structures and regulatory enzymes for internalizing the sperm nucleus, directing the fusion of the two nuclei, setting up the rapid sequence of cell divisions that follow, and even establishing the patterns of division and subsequent embryonic events. Following fertilization, most protostomes develop rapidly into a fully functional feeding larva or juvenile, and the oocyte must take care of all the needs of the developing embryo until it is capable of feeding on its own. Thus, in addition to the mechanical and regulatory apparatus, the oocyte generally must contain large nutrient stores in the form of lipid- and protein-rich yolk.
Some protostomes regularly engage in sexual reproduction, yet do not require the development of both sperm and oocytes. In many of these cases, the species are technically gonochoristic, but males are rarely or never produced. However, if the offspring develop from true oocytes, with the re duction of chromosome number, even without subsequent fertilization, this is a form of sexual reproduction. If no true oocytes are formed by the reductional division of the chromosomes, the reproduction is asexual, even though the progeny cells look like oocytes. Whether sexual or asexual, this type of reproduction by female-only species is known as parthenogenesis. Among protostomes, some insects (phylum Arthropoda) are well known for their parthenogenetic capabilities.
Copulation, spawning, and fertilization
Because gametes are capable of limited or no motility relative to the vast habitat in which the animals live, each species must have a way of bringing the sperm and oocytes close to each other so that fertilization can occur. The mechanisms for doing this are numerous, and involve a dazzling diversity of behavioral and anatomical modifications across the spectrum of protostome life. Despite the diversity, all can be grouped generally into two broad categories, copulation and spawning.
Copulation involves various mechanisms by which one member of a mating pair physically introduces sperm into the body of its partner. In hermaphroditic species, this insemination is usually reciprocal. The precise mechanism of insemination varies among protostome groups, as does the site of insemination. Many snails (phylum Mollusca), especially marine prosobranchs, possess a large penis that can extend all the way out of the shell of the male and into the mantle cavity of the female, depositing sperm directly in the genital opening. Many male crustaceans and insects (phylum Arthro-poda) have complex exoskeletal structures, derived from specific appendages or body plates, which lock mechanically with complementary plates surrounding the genital opening of the female. Some protostomes transfer special packets of sperm, known as spermatophores, to their mating partner, so that the individual sperm can be released into the female's system some time after copulation has ended. For example, male squids (phylum Mollusca) use a modified arm to place a loaded spermatophore inside the mantle cavity of a female. Some hermaphroditic leeches (phylum Annelida) actually spear their mating partner through the skin with a dartlike sper-matophore, which slowly injects the sperm through the body wall following copulation. In almost all cases, whether by sperm or spermatophore transfer, copulation is followed by internal fertilization, and at least some degree of internal development. The benefits of internal fertilization and development are especially great in terrestrial environments, so virtually all terrestrial protostomes copulate. Likewise, the freshwater environments are not generally hospitable for gametes and embryos, so most freshwater protostomes are cop-ulators, although there are some exceptions.
The vast majority of marine invertebrates are broadcast spawners, meaning that they broadcast their gametes freely into the open seawater. A few freshwater species, such as the well-known invasive zebra mussel (phylum Mollusca) also engage in broadcast spawning. In most cases of broadcast spawning, both the sperm and oocytes are spawned so that fertilization is external. But in a few groups, such as some
Earthworm Hydrostatic Skeleton coelom (containing water) circular muscles longitudinal muscles epidermis cuticle
clams and other bivalve mollusks, only the males spawn, leaving the adult females to draw sperm into their bodies for internal fertilization. Following internal fertilization, many species brood their young for some period of time, either internally, as in some snails, or externally in egg masses, as in some decapod crustaceans. Even among broadcast spawners with external fertilization, some species take up embryos or larvae from the open water and brood them internally, or brood them externally on the body surface.
For most protostomes, sexual reproduction is highly periodic, so copulatory and spawning behavior are also periodic. Focusing all gamete-releasing into defined periods of time is yet another way that the fully formed gametes can achieve higher rates of success in encountering one another. Among terrestrial and freshwater species, the periodicity is generally annual, occurring only at certain seasons of the year. The same may be true for marine species, particularly in near-shore environments, where seasonal runoff of rainwater from rivers provides seasonal cues for sexual activity, as well as seasonal surges in nutrients to feed the resulting larvae. In other marine environments, reproductive periodicity is often influenced more by lunar or tidal rhythms, and so may occur in monthly rather than in annual cycles.
Following successful fertilization, the zygote must commence formation of a multicellular embryo, a process known
Locomotion in different animals: A. Squid propulsion; B. A snail's muscular foot; C. Leg extension in arthropods. (Illustration by Patricia Ferrer)
as embryogenesis. The actual establishment of multicellu-larity from the unicellular zygote involves a process known as cleavage. Cleavage involves more than simple cell division, for example, mitosis. True multicellularity involves the division of labor among cells, so each cell has to take on a special identity and developmental fate shortly after becoming independent of its progenitor cell. The process of acquiring a distinctive function is known as differentiation, and acquiring a specific developmental fate is known as determination. Protostomes generally undergo differentiation and determination very early in development, in many cases at the very first cell division of cleavage. This is easily visible under a standard microscope for some phyla, but is hidden from view by the highly modified cleavage patterns of insects, spiders, and some other arthropods.
The first thing that distinguishes protostomes from deuterostomes is this early determination. Thus, protostomes are often said to undergo determinate cleavage, or mosaic development, in contrast to the indeterminate cleavage, or regulative development, of deuterostomes. These two cleavage patterns are so different that they can be distinguished easily with a microscope. The determinate cleavage of protostomes results from a plane of cell division, usually visible after the second division, that cuts diagonally across the original zygote axis, thus compartmentalizing different regulative and nutritive chemicals in each of the resulting cells. This is referred to as spiral cleavage, since the cells dividing diagonally appear under the microscope to spiral around the original axis. In contrast, the indeterminate cleavage of deuterostomes results from planes of cell division that cut alternatively lon
gitudinally along the zygote axis, then transversely across the axis, thus leaving each resulting tier of cells with similar regulative and nutritive chemicals. This is referred to as radial cleavage, since the cells dividing at alternating parallel and right angles to the original axis appear under the microscope to radiate in parallel planes from that axis. The most important thing is not whether the resulting cell masses appear to spiral or to radiate, but that the spiraling cells of the protostomes show determination of specific germ layers as early as the first cell division, and almost universally by the third. Thus, at the very earliest stages of cleavages, specific cells of protostomes have already been determined to a fate of forming one of the three germ layers.
Within these basic functional forms of cleavage, there are many variations in the specific spatial configurations and the extent of cell division. Most protostomes undergo some type of holoblastic cleavage, in which the two daughter cells become completely separated, each with its own complete cell membrane. This type of cleavage may be described as either equal cleavage or unequal cleavage, depending on whether the daughter cells are equal in size. Most protostomes exhibit unequal holoblastic cleavage. In all these, the large cells are called macromeres, and they usually form the endoderm and the mesoderm. Small micromeres at the other end of the em bryo generally form the ectoderm. Some animals have me-someres of an intermediate size, which may contribute to either the ectoderm or the mesoderm, depending on the species. In contrast, many arthropods with very large, heavily yolked oocytes undergo a form of incomplete cleavage known as superficial cleavage, in which the incompletely divided daughter cells ultimately reside as a layer surrounding a shared yolk mass. This appears similar to the meroblastic cleavage seen in large yolky eggs of birds and reptiles, but true superficial cleavage in arthropods begins with multiple divisions of the nuclei prior to the division of the cytoplasm.
Blastulation, gastrulation, and coelom formation
During and after cleavage, embryonic development continues with a series of rearrangements among the cells and cell layers. In the first of these, known as blastulation, the cells in the solid mass resulting from cleavage simply arrange themselves in preparation for the establishment of the spatially segregated germ layers. Blastulation begins during the middle-to-late stages of cleavage, and varies in the degree of layer organization. The final blastula stage of most protostomes is a solid mass of cells, known as a stereoblastula. Typ ical examples of this can be seen among many marine mollusks and annelids. In some protostomes, the blastula stage, known as a coeloblastula, is a hollow ball of cells that are arranged in a single layer around the central cavity, known as a blastocoel. The ribbon worms (phylum Nemertea) are not considered coelomates by most biologists, and therefore are not technically protostomes. However, they undergo typical spiral cleavage and develop through a coeloblastula stage and exhibit other protostome characteristics, so most biologists consider them to be closely related to the protostomes.
After blastulation, the blastula is now ready to undergo a critical process in which the three embryonic germ layers are established. This process is known as gastrulation, since it is characterized by the internalization of the endodermal cells to form the archenteron, which is the ancestral gastrointestinal tract. Gastrulation involves a specific set of cell movements that vary widely, depending on the animal group. These mechanisms range from invagination, to inward migration, to inward growth and proliferation. The end result, however, is the same. The endodermal cells are now internal, forming the archenteron gut tube, while the mesodermal cells take up residence between the endoderm and the ectoderm, which comprises cells that remained on the outside of the embryo. Regardless of how the gastrulation process takes place, the embryo is left with an opening to the outside; this opening, the blastopore, is encircled by a rim that forms the boundary between endoderm and ectoderm, and will develop into an opening into the gut in the adult animal. The precise nature of the opening is the second major defining attribute of the protostomes, in which the fate of the blastopore is to form the adult mouth. Conversely, the fate of the blastopore in deuterostomes is to form the anus.
Shortly or immediately after gastrulation is complete, protostomes form their body cavity, the coelom. By definition, a true coelom is always a body cavity within mesodermal tissue. The mechanism by which the coelom is formed is the third primary distinction between protostomes and deuteros-tomes. In most deuterostomes, the coelom forms by out-pocketing from the original archenteron, a process known as enterocoely, since the coelomic cavities are thus derived directly from embryonic enteric cavities. In protostomes, the coelom forms from a split in the previously solid mass of mesodermal cells, a process thus known as schizocoely. There are some exceptions to this rule, but it holds true in most cases. Some protostomes lack a coelom as adults, but even these typically go through a coelomate embryonic and/or larval stage.
The gastrula stage is technically the last stage of embryonic development, so every stage following, up to the adult, is postembryonic. Many protostomes undergo postembryonic development that is direct. In these cases, the gastrula develops directly into a juvenile, which is typically a miniature, but sexually immature, version of the adult. The juvenile then has simply to grow and mature to become an adult. The vast majority of protostomes take a very different approach, engaging in a more complex pattern known as indirect development.
This involves the development of the gastrula into some sort of distinctive larva, which is both immature and quite different from the adult. Typically, larvae have functions in the life history that are critical to the species, yet differ from that of the adult. In most marine protostomes, the primary function of the larval form is to provide for the dispersal of the species to colonize new habitats. Larvae are generally well suited for this since they are very small, and thus easily carried freely floating in the water as plankton. This planktonic dispersal of larvae is especially well developed among the marine annelids, mollusks, and crustaceans, but also occurs in the minor protostome phyla, such as Echiura and Sipuncula.
Larvae occur in many types, depending on the phylum and species, and each of these types has been given a specific name. In the simplest forms, such as with marine mollusks and annelids, the trochophore is little more than a gastrula with bands of cilia for swimming. At the other end of the spectrum of complexity, marine crustaceans may go through a succession of anatomically distinct larval stages, such as the nau-plius, zoea, or megalopa. Larvae of all groups rely on considerable nutrients as they disperse and develop, but they acquire them in different ways. Depending on the species, they are either planktotrophic, feeding on plankton as they drift, or lecithotrophic, relying on stored yolk material obtained from the mother. Regardless of the number or type of larval stages, each species will eventually undergo metamorphosis, a dramatic change of morphology into the adult form. In some species, there is an intermediate juvenile stage, so that postembryonic development is mixed, having indirect and direct components. Insects are especially variable in this regard. In the case of freshwater insects, the larval and juvenile stages are often the dominant stage in the life cycle. In some of these, such as caddisflies and mayflies, the larvae may live for one to several years, whereas the adult lives for only days. In some terrestrial insects, such as cicadas, the larvae may live up to 17 years, with the adults living only a few weeks. In cases such as these, the larva actually defines the species ecologically, and the adult is simply a short-lived stage necessary for sexual reproduction.
The final stage of postembryonic development is sexual maturation. This is preceded by the final development of critical body parts, and even of the fundamental body framework, as in the segmentation, or metamerism, of annelids and arthropods. Sexual maturation may occur immediately following embryonic development, or may be arrested for many years. Many protostomes undergo sequential cycles of sexual maturation, growing gonads and/or gametes during certain seasons, and completely losing them in others. During non-reproductive periods, such an animal may appear to be a large juvenile. Notable among these are the many marine poly-chaete worms (phylum Annelida) that lack distinct gonads, but whose gametes form from mesodermal peritoneal cells lining the coelom only when the proper environmental cues induce them to transform.
The most important variation among postembryonic on-togenetic strategies involves the degree to which animals can
modify their cells to perform various functions, that is, to change the developmental fate of their cells. Some species, especially among the lower metazoans, have cells that are de-velopmentally plastic, in that the cells retain the ability to become, or to form through cell division, many different cell types, depending on the needs of the animal. Other animals tend to have cells with very limited developmental plasticity. For example, a skin cell in adult humans can only produce other skin cells; any departures from this would result in a malformation, such as skin cancer. No coelomates have as much developmental plasticity as do lower metazoans such as sponges and cnidarians. However, as a general rule among the coelomates, protostomes tend to have a lower degree of developmental plasticity than do deuterostomes.
Asexual reproduction is defined as any reproduction that does not involve meiosis, which involves genetic recombination and reduction of chromosomal number during cell division. Asexual reproduction is not as common among protostomes as it is among the lower metazoans such as sponges, hydroids, and flatworms. However, it is quite common and well developed among some oligochaete and poly-
chaete annelids. In these, it usually involves some type of fission, in which the adult body splits into two or more pieces, each of which reconstitutes the missing parts. Although rare, asexual reproduction does occur among some groups, such as the asexual parthenogenesis of some aphids and some freshwater snails. Another type of asexual reproduction, the polyembryony of some wasps, involves a rare asexual proliferation of embryonic masses to form several individuals.
Brief summaries of the primary reproductive and developmental strategies of each generally recognized protostome phylum follow. However, the variations are great, and the short summaries below are intended only to place each phylum within the overall context of strategies and processes discussed above.
The segmented worms are quite variable in their reproductive and developmental strategies. Most of this variation occurs along class-specific lines. For example, the vast majority of polychaetes (including clamworms, scaleworms, and fanworms), most of which are marine, are gonochoristic. In marked contrast, clitellates (earthworms, leeches, and their relatives), many of which are terrestrial and freshwater, are hermaphroditic. Most polychaetes are broadcast spawners; clitellates generally exchange sperm by mutual insemination, through copulation or spermatophore insertion. Even among the copulators, most annelids leave their young at an early embryonic stage. Some polychaetes and leeches engage in external brooding, with the young developing either directly on the outside of their body or in their burrows or tubes. Most poly-chaetes possess rather simple gonads, and some have dramatic synchronized spawning events that culminate in rupture of the body wall to release the gametes into the surrounding seawa-ter. Clitellates generally have complex gonads, commensurate with their copulatory behavior and deposition of offspring within cocoons in freshwater and terrestrial environments. Cleavage is generally holoblastic, and the typical spiral pattern may be obscured by modifications in some clitellates. Most annelids of all classes produce a coeloblastula that undergoes gas-trulation by an invagination process. Marine polychaetes develop into trochophore larvae that are similar to those of mollusks, possibly indicating a phylogenetic relationship. Clitellates generally undergo direct development. Asexual reproduction does not occur in most annelids, but is utilized by a few polychaetes and some freshwater oligochaete clitellates.
The exclusively tropical and terrestrial velvet worms are gonochoristic, with well-developed gonads and a copulatory behavior that involves the deposition of spermatophores by the male externally, onto the body of the female. Fertilization is internal, following movement of the sperm directly through the body wall into the female. Cleavage is holoblastic in most species, and superficial in others, but always resembles that of various arthropods. Some species brood their young internally. Postembryonic development is direct, and there is no record of asexual reproduction.
Water bears are mostly gonochoristic, but a few hermaphroditic species are known. They have rather simple go-nads, and their copulation results in internal or external fertilization, with zygotes retained within the shed cuticle of the mother. They go through holoblastic cleavage leading to a coeloblastula, and followed by direct development into a juvenile. The juvenile is smaller than the final adult, but consists of the same number of cells. Thus, maturation involves growth of cells without new cellular reproduction. Sexual parthenogenesis occurs in some, but asexual reproduction is unknown.
As the overwhelmingly largest phylum in the animal kingdom, arthropods exhibit a diversity of reproductive and developmental strategies that cannot be easily summarized. Most are gonochoristic, but there are exceptions. Most arthropods are insects, and the primarily terrestrial nature of this group has resulted in reproductive systems and development patterns that have adapted to the desiccating effects of this environment. In all habitats, including the marine environment where the majority of crustaceans live, copulation is the general rule. This occurs by numerous and varied means. Brooding and maternal care is also well developed in the phylum, with the complex societies of wasps and ants being at the very pinnacle of its development in the animal kingdom. The gonads are generally tubular throughout the phylum, but the tubes are generally regionally modified to perform a variety of sophisticated functions. Sperm and oocytes typically develop inside complex follicles from which they derive materials prior to fertilization, which is typically internal in all environments. Cleavage may be holoblastic, as in most spiders, millipedes, and crustaceans. Insects and some other groups, however, have such large yolk reserves that holoblas-tic cleavage is impossible, and these exhibit superficial cleavage around the central yolk mass. Marine crustaceans usually have planktonic larval forms, and larvae or nymph juveniles of many insects are important feeding stages in the life cycle. Asexual reproduction occurs in very few species, and is limited to asexual parthenogenesis and some reported cases of asexual division of embryos.
Reproduction is extremely varied in this second largest of all animal phyla. Among the large classes, examples of gono-chorism and hermaphroditism abound, with striking sexual dimorphism in some of the former. Broadcast spawning or copulation occurs among the marine species, but copulation is more common among those of terrestrial and freshwater environments. Several species engage in sometimes-sophisticated brooding behavior. The reproductive systems are generally quite sophisticated, especially among the snails that lay complex egg masses. The vast majority of mollusks undergo holoblastic cleavage, but squids and octopuses have large yolky eggs that cleave incompletely, similar to the eggs of birds and reptiles. Blastulae may be either hollow or solid, depending on the species. Most species produce a trochophore larva, and in many, especially marine snails and bivalves, this is followed by a distinctive veliger that begins to acquire juvenile characteristics leading up to settlement. Some species brood their tro-chophores and veligers. The most extreme example of this involves freshwater unionid clams, which brood their modified glochidia veligers prior to releasing them to become parasites on fish. Asexual reproduction is rare, and involves only a few known cases of asexual parthenogenesis. Some populations in these cases have no males at all.
The spoon worms are gonochoristic, and have simple go-nads similar to those of annelids, to which they are closely related. With few exceptions, they are broadcast spawners, whose unequal holoblastic cleavage leads to an annelidlike tro-
chophore larva. One oddity among certain spoon worms involves a tiny dwarf male living inside the genital sac of the dramatically larger female, resulting in internal fertilization. Asexual reproduction is not known to occur.
The peanut worms are gonochoristic, with rather simple gonads from which they broadcast spawn their gametes into the open seawater. Their reproductive systems are similar to those of many polychaetes, but their development is more like that of mollusks; they are thought to be related to both, but more closely to the mollusks. Holoblastic cleavage leads through gastrulation to a trochophore larva, similar to that of mollusks. Some species have more than one larval stage. Asexual reproduction has been reported to occur, but has been poorly studied.
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David Bruce Conn, PhD
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