184 STRUCTURE OF THE MOLLUSCA. 



from the mantle, and almost always a compressed fleshy 

 foot. Fi^. 5. 



V. Brachiopoda. Branchiae not distinct from the mantle ; 

 two ciliated filiaments or arms in place of a foot. Fig. 6. 



VI, Tunic AT A. The body enclosed in a cartilaginous 

 hag ; branchiae not distinct ; no special organs of motion. 

 Fig. 7. 



In this arrangement, the general form of the body of these 

 animals, being in relation with the greater or less complex- 

 ity of their internal organization, is assumed as a basis. 

 Thus, some have a distinct head, figs. 1, 2, 3, 4, 8; while 

 in others there is no such part separated or distinguishable 

 from the rest, figs. 5, 6, 7, 9. Among the Mollusca fur- 

 nished with a head, there are some, the Cephalopoda, fig. 1, 

 which have around the mouth long appendages subservient 

 to locomotion; others, the Pteropoda, fig. 2, have wing-like 

 appendages or fins on the two sides of the body; and in 

 others, the Gasteropoda, fig. 3, the lower part of the body 

 forms a kind of muscular foot, by which the animal crawls. 

 Of the Mollusca of which the head is not distinct, some, 

 the Acephala, or Acephala Lamellihranchiata, fig. 9, have 

 the respiratory organs in the form of four thin plates, two 

 on each side, within the mantle, and not adherent to it; 

 others, the Brachiopoda, fig. 6, have two twisted and cili- 

 ated appendages or arms; and others, the Tunicata, fig. 7, 

 have no organs of motion, but are shut up in a leathery 

 covering or bag. 



The general features or characters of the Mollusca given 

 above, may now be a little more particularly described. 



Their nervous system is composed of nervous filaments 

 connected with a small number of ganglia, or little nervous 

 masses, fig. 9, r s, dispersed in different parts of the body. 

 The principal mass formed by these ganglia, which may be 

 compared to the brain in the higher animals, fig. 8, p, is 

 placed transversely over the oesophagus, or gullet, which it 

 envelopes with a nervous ring or collar. But there is no- 

 thing in the Mollusca that can be compared to the spinal 

 marrow, which in the vertebrated animals is cased in an 



