Unit Five. Evolution of Animal Life
Even though acoelomates and pseudocoelomates have proven very successful, the bulk of the animal kingdom consists of coelomates. Coelomates have a new body design that repositions the fluid. What is the functional difference between a pseudocoel and a coelom, and why has the latter kind of body cavity been so overwhelmingly successful? The answer has to do with the nature of animal embryonic development. In animals, development of specialized tissues involves a process called primary induction, in which the three primary tissues (endoderm, mesoderm, and ectoderm) interact with each other. The interaction requires physical contact. A major advantage of the coelomate body plan is that it allows contact between mesoderm and endoderm, so that primary induction can occur during development. For example, contact between mesoderm and endoderm permits localized portions of the digestive tract to develop into complex, highly specialized regions like the stomach. In pseudocoelomates, mesoderm and endoderm are separated by the body cavity, limiting developmental interactions between these tissues.
The only major phylum of coelomates without segmented bodies are the Mollusca. The mollusks are the largest animal phylum, except for the arthropods, with over 110,000 species. Mollusks are mostly marine, but occur almost everywhere.
Mollusks include three general groups with outwardly different body plans. However, the seeming differences hide a basically similar body design. The body of mollusks is composed of three distinct parts: a head-foot, a central section called the visceral mass that contains the body’s organs, and a mantle. The foot of a mollusk is muscular and may be adapted for locomotion, attachment, food capture (in squids and octopuses), or various combinations of these functions. The mantle is a heavy fold of tissue wrapped around the visceral mass like a cape, with the gills positioned on its inner surface like the lining of a coat. The gills are filamentous projections of tissue, rich in blood vessels, that capture oxygen from the water circulating between the mantle and visceral mass and release carbon dioxide.
The three major groups of mollusks, all different variations upon this same basic design, are gastropods, bivalves, and cephalopods.
1. Gastropods (snails, like the one shown in figure 19.15a, and slugs) use the muscular foot to crawl, and their mantle often secretes a single, hard protective shell. All terrestrial mollusks are gastropods.
2. Bivalves (clams, oysters, and scallops) secrete a two-part shell with a hinge (figure 19.15b), as their name implies. They filter-feed by drawing water into their shell.
3. Cephalopods (octopuses, like the one shown in figure 19.15c, and squids) have modified the mantle cavity to create a jet propulsion system that can propel them rapidly through the water. In most groups, the shell is greatly reduced to an internal structure or is absent.
Figure 19.15. Three major groups of mollusks.
(a) A gastropod. (b) A bivalve. (c) A cephalopod.
The Phylum Facts illustration walks you through the characteristics of mollusks, including a unique feature of mollusks, the radula, which is a rasping, tonguelike organ. With rows of pointed, backward-curving teeth, the radula is used by some snails to scrape algae off rocks. The small holes often seen in oyster shells are produced by gastropods that have bored holes to kill the oyster and extract its body.
In most mollusks, as stated earlier, the outer surface of the mantle also secretes a protective shell, partially cut away in the facing Phylum Facts illustration. The shell consists of a horny outer layer, rich in protein, which protects the two underlying calcium-rich layers from erosion. The inner layer is pearly and is used as mother-of-pearl. Pearls themselves are formed when a foreign object, such as a grain of sand, becomes lodged between the mantle and the inner shell layer of a bivalve, including clams and oysters. The mantle coats the foreign object with layer upon layer of shell material to reduce irritation. The shell serves primarily for protection, with some mollusks withdrawing into their shell when threatened.
Key Learning Outcome 19.8. Mollusks have a coelom body cavity but are not segmented. Although diverse, their basic body plans include a foot, the visceral mass, and a mantle.