The well-developed circulatory system con- 

 sists of a heart and definite blood vessels, 

 which lead to the major organs. However, 

 the molluscan system is of the open type 

 (p. 319). In most of the organs, the circula- 

 tory fluid, or hemolymph, passes directly 

 through the tissue spaces, rather than through 

 clearly delimited capillary channels. Usually 

 the respiratory organs are gills. And finally, 

 the sensory-neuromuscular system is highly 

 organized, especially in the squids, octopuses, 

 and other cephalopods. The central nervous 

 system, typically, consists of three pairs of 

 interconnected ganglia: (1) the cerebral gan- 

 glia, situated anteriorly near the mouth; 

 (2) the pedal ganglia, imbedded in the foot; 

 and (3) the visceral ganglia, situated poste- 

 riorly in the body. The peripheral system, 

 on the other hand, is represented by the 

 numerous sensory nerve fibers coming in 

 from the receptors and the motor fibers going 

 out to the musculature, especially that of the 

 foot. 



All Mollusca have tactile and chemical 

 receptors, and many possess statocysts (Fig. 

 23-7), which enable the animal to determine 

 its orientation and equilibrium. Simple light- 

 sensitive "eyespots" are present in some (for 

 example, scallops), and a few mollusks (for 

 example, squids and octopuses) have highly 

 developed eyes, which achieve very accurate 

 true vision. In fact, the eye of the mollusk is 

 structurally very similar to the vertebrate eye. 

 However, the manner of embryonic origin — 

 from a folding of the surface ectoderm, rather 

 than from an outgrowth of the brain — indi- 

 cates that the two organs cannot be homolo- 

 gous. In fact, this is frequently cited as 

 a striking example of convergent evolution. 



Other Molluscan Features. Clams, oysters, 

 and other bivalves feed upon semimicroscopic 

 particles and organisms. These are carried to 

 the small mouth by a copious (3 quarts per 

 hour for an average oyster) stream of water 

 brought into the shell by an incurrent siphon 

 and passed out by an excurrent siphon. The 

 mouth, in such cases, is not equipped with 

 any kind of chewing or biting structures; but 



The Animal Kingdom - 651 



almost all other mollusks possess several rows 

 of minute chitinous teeth, collectively called 

 the radula, bordering the mouth opening. 

 The radula may serve for scraping algae and 

 other encrusting organisms from a rocky sur- 

 face, as in the case of the sluggish creeping 

 Chiton (Fig. 32-29); or it may serve for rasp- 

 ing and boring into the shell of another 

 organism or into the wooden bottom of a 

 ship, as in the case of one very serious marine 

 pest, the "shipworm," Teredo. The mouth, 

 in squids and octopuses, is also equipped 

 with two powerful horny beaks. These can 

 kill a captured fish or other prey and can 

 tear the food into pieces suitable for swal- 

 lowing. 



Molluscan shells vary widely as to form 

 and composition; and some forms (for exam- 

 ple, slugs) do not have any shell. Typically, 

 the shell — whether univalved, bivalved, or 

 multivalved — is composed of a dense calcare- 

 ous material superimposed upon a delicate 

 chitinous framework. Usually it forms a hard 

 external protective covering for the animal. 

 But in the squid, the shell is reduced, and it 

 lies internally. In fact, the delicate, chitinous, 

 quill-like shell of the squid is commonly 

 referred to as the "pen" (Fig. 32-28). 



Many molluscan shells, of course, are very 

 beautiful. They display a wide variety of 

 lovely forms; and the mother-of-pearl (nacre) 

 layer, which lines the inner surface of the 

 shell, is often richly colored in various iri- 

 descent hues. Pearllike bodies are apt to 

 form whenever a grain of sand or other small 

 source of irritation becomes lodged on the 

 outer surface of the mantle, where the nacre 

 of the shell is being secreted. But pearls of 

 real value, in which the layers of nacre are 

 exceedingly hard and regular, are produced 

 rarely, and only in certain kinds of oysters. 

 The shell of the "many-chambered nautilus" 

 actually represents a number of shells, formed 

 year after year by the same animal, and fused 

 into a spiral series. The successive shells are 

 larger and larger. Thus each year the nautilus 

 builds itself a "more stately mansion," but 

 always it remains burdened by its older 



