SUB-KINGDOM V.—M0LLU8GA. 



CHAPTER XIX. 



Sttb-kingdom Mollusca. — The Mollusca (Lat. mollis, 

 soft), as implied by their scientific name, are mostly soft- 

 bodied animals, but their popular name of " shell-fish " ex- 

 presses the fact that their soft body is usually protected by 

 an external skeleton or " shell." All the Mollusca are fur- 

 nished with a distinct alimentary canal, which is completely 

 shut off from the general cavity of the body (Fig. 75, d). 

 There is sometimes no distinct blood-circulatory apparatus ; but, 

 when there is, its central portion (i. e., the heart) is placed upon 

 the dorsal aspect of the body. The chief peculiarity, however, 



Fig. 75.— Diagram of a Mollusk. a Alimentary canal ; K Heart; / Foot; n Cerebral gan- 

 glion; n' Pedal ganglion ; n" Parieto-splanchnio ganglion. 



of the Mollusca is found in the nature of the nervous system. 

 In the lower forms (Fig. 76, 3 d), the nervous system consists 

 essentially of a single ganglionic mass, giving off filaments in 

 various directions. In the higher Mollusca (Fig. 75, n), the 

 nervous system consists of three scattered ganglia, united to 

 one another by nervous cords. One of these ganglia is placed 

 above the gullet or oesophagus, and is known as the "supra- 

 oesophageal " or " cerebral " ganglion. A second supplies 



