Choanocytes

pinacocytes flagellum c. J„L

collar cell body

Figure 5.3 The components of sponges. a: Connective elements. Spicules are mineralized connective elements made either of calcite or silica. Certain sponges lack spicules but are held together by networks of a fibrous protein, spongin. [From Storeretal. (1979)] b: Cell types. c: The choanocyte filters microorganisms and other food items from the stream of water driven through its collar by the flagellum. Solid arrows (on collar of choanocyte) indicate direction of movement of captured items of food [b and c from Hickman and Hickman (1992)]

ing networks of vessels that permeate the sponge's "body." Nevertheless, it still is hard to say that the shape of a sponge's "body" results from a developmental process typical of a metazoan animal, with its complicated series of programmed growth, differentiations, and contingent developmental events. The difference is illustrated nicely by the ways animals and sponges recover from a disruption of development. Take a sponge, scramble it into its constituent cells, leave the disordered mess in a dish of seawater, and after a few days its cells will reassociate in particular ways, as if Humpty-Dumpty put himself back together again. Animals can do something similar: cut the limb off a developing chick, for example, and soon a new limb will regenerate. The similarity is only superficial, however: re-association of sponge cells is not regeneration. In the sponge, the very cells that were separated from their neighbors soon find the same or similar neighbors to reassociate with. In limb regeneration, the developmental process of limb formation is done all over again, but with a whole new set of cells going through the same process of growth and differentiation as the original limb cells underwent.

Among animals, development is tightly constrained by genetic programs, but not completely so: the embryo's environment during development also has effects. In the jargon of developmental biology, body form is the result of both genetic (inherent) and epigenetic (environmental) factors. Among the Metazoa, it is clearly the genetic control that dominates, as is reflected in the high degree of fidelity of the body plan: a dog's offspring will always be puppies, recognizable as puppies irrespective of what environment they develop in. Environmental influences surely play a role: a period of starvation during development may make the body smaller or more slender, or it may alter the way the brain develops, but even dog embryos starved during development still are born as puppies and grow into dogs.

Among sponges, however, it seems to be the epigenetic factors that are dominant, while the genetic control is relatively weak. For example, the basic growth form of sponges of the genus Haliclona is a ramifying series of the sieve-like tubular elements typical of the asconoid sponges. However, the growth forms of Haliclona vary widely with the types of water flow they experience as they grow. When Haliclona grow in still waters, the branches are long, thin, and few in number. When Haliclona grow in flowing water, the branches are short, squat, and numerous, at the extreme merging into the stout growth form typical of a leuconoid sponge. One can even change an individual sponge's body form simply by moving it from one habitat to the other. Indeed, what form a sponge takes seems to be less strongly determined by the kind of sponge it is (by whatever genetic legacy directs its development) than by the environment in which it lives.

Body Plans of Coelenterates and Corals In contrast to the sponges, coelenterates are unambiguously animals. As such, genetic factors assert themselves more strongly during development than they do in sponges. Like sponges, coelenterates start as a simple cell and grow into a hollow tube closed at one end (Fig. 5.4). Development of the digestive tract stops at the archenteron stage (although it is called the coelenteron in the adult). The coelenteron opens to the outside through a single opening, which serves as both mouth and anus, and is surrounded by a two-layered body wall. The inside layer of cells, lining the coelenteron, is the endoderm, while the one facing outward is the ectoderm.

Unlike sponges, in which the "body plan" of one type seems to merge almost imperceptibly into the "body plans" of other types, the coelenterates can be differentiated pretty reliably by their body plans. For example, coelenterates exhibit a life history known as alternation of generations. The basic diploblastic body plan of coelenterates exists in two forms: an asexual polyp, typically sedentary, frequently nonreproductive or reproducing by budding of the parent animal; and a sexual medusa, a swimming form with sexual organs. The life history of most coelenterates alter-

round the closed primitive gut, the archenteron. [From Storeretal. (1979)]

nates between these two stages: sexually reproducing medusae give rise to embryos that develop into polyps, which grow and from which medusae eventually bud again. The classes of coelenterates can be differentiated by which of these two life stages dominates the life cycle. Among the Anthozoa ("flower animals"), which include the sea anemones, the polyp stage dominates, and the medusa stage is relegated to fairly insignificant animals, in some species being lost altogether. Among the Scyphozoa ("cup animals"), which include the free-swimming jellyfish, the dominant stage is the medusa. These forms exist even in the face of wide variation in environmental conditions, suggesting the genetic factors in development are relatively robust in comparison with the epigenetic factors.

Coelenterates have not been entirely tamed by the genetic bridle, though. Some coelenterates, most notably the corals, exist as colonial organisms whose "body" is made up of a more or less obligatory association of individual polyps. Development of the individual polyps might be under fairly tight genetic control, but the development of their association seems to be less tightly constrained. Thus, one sees a very strong environmental component to the growth forms of corals, similar to the pattern seen in sponges, even if the individual polyps retain characteristic patterns of growth. Corals like Millepora, for example, ramify as they grow, resulting in arborescent forms reminiscent of the sponge Haliclona. Just as in Haliclona, the corals residing in still waters develop long, thin branches, while those in rougher waters are stouter, squatter, and more highly branched. This is a common feature of many types of corals, and again, it seems that epigenetic factors dominate the morphology of the association, if not the individuals that constitute it.

Both sponges and corals, then, build structures that seem to arise without undue complicating influence of genetic constraints on development. They seem to fit the bill nicely for an examination of how metabolic energy interacting with energy in the environment can result in permanent, tangible structures—edifices, furthermore, that do physiological work. Admittedly, we are stretching the definition of structure a bit far for the sponges: when you look at a sponge, are you seeing an animal's body, or are you seeing a structure built by a colony of cells? I'm simply going to adopt the latter view and ask your indulgence: I really have no good reason for doing so other than it helps me make my argument.

structure known as a corallite. A polyp together with its associated corallite form a modular unit known as a zooid. A coral "animal" consists of all the polyps and their associated corallites;in other words, it exists as a collection of the modular zooids. Growth of the coral occurs either by growth of a zooid, usually by the extension of the corallite, or by multiplication of the zo-oids.

Among sponges, modular growth arises because the spicules of sponges often are organized into polygonal

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