468 The Animal Kingdom 



and out of the heart, and an extensive series of blood sinuses, the 

 hemocoel. The circulatory fluid has as its respiratory pigment hemo- 

 cyanin or hemoglobin in a few cases. 



The respiratory system is highly variable. In some, gills are pres- 

 ent, in others a lunglike structure, and yet in others the mantle or 

 epidermis performs as an organ of respiratory exchange. The excretory 

 system consists of a kidney composed of numerous metanephridial 

 tubules. 



Like all the other systems of the molluscs, the nervous system 

 varies greatly in the different classes. Basically it is composed of scat- 

 tered ganglionic masses connected by nerve tracts. Sense organs are 

 present in varying degrees of development among the different species. 

 The squids and octopuses possess eyes comparable in complexity to the 

 vertebrate eye. 



The sexes are usually separate ; development may be direct or 

 through a modified trochophore larva, the veliger. Cleavage is deter- 

 minate. 



The Classes of Mollusca. — There are five classes which are dis- 

 tinguished from one another by the character of the foot and mantle. 



Class I. Aniphineura. Forms possessing elongated bodies usually with a 

 dorsal shell consisting of eight plates with a large oval, ventral foot. All marine 

 forms. Example : chitons. 



Class II. Scaphopoda. Mantle and shell tubular and curved, open at both 

 ends, strictly marine forms, with cone-shaped foot. Example : Dentalium, the 

 tooth shell. 



Class III. Gastropoda. Usually with an asymmetrical mantle and shell, 

 distinct head bearing tentacles. Foot large and flat ; some forms without an ex- 

 ternal shell. Marine, fresh-water, and terrestrial dwellers. Examples : snails, 

 slugs, and limpets. 



Class IV. Pelecypoda. Visceral mass enclosed in a double-hinged shell, 

 mantle under the shell, foot hatchet-shaped. Both marine and fresh-water forms. 

 Examples : clams, oysters, mussels, shipworm. 



Class V. Cephalopoda. Mantle may or may not secrete a shell which is 

 either internal or external, large head with many tentacles, well-developed eyes, 

 mouth with horny jaws and radula. All marine forms. Examples : squids, octopuses, 

 nautilus. 



THE CLASS AMPHINEURA 



This class, which includes the chitons (Fig. 156,i), shows the 

 typical molluscan characteristics in the least modified form. These 

 strangely armored molluscs with their eight dorsal plates are found in 



