ORGANS OF DIGESTION. 



dition of the animal matter on which they commonly sub 

 sist. 



FOURTH SECTION. 



Digestive Organs of the Cydo-gangliated or Molluscous 



Classes. 



The low development of all the organs of sense and loco- 

 motion throughout the molluscous classes, compared with 

 those of the articulata, renders them less able to select and 

 overcome the higher forms of animals as prey, and requires 

 their digestive apparatus to be adapted for a more coarse and 

 varied kind of food. The slow moving or fixed animals of 

 this division, subsisting on organic matter in a lower con- 

 dition of development, and consequently more remote from 

 their own nature, possess a more extended and complicated 

 alimentary canal, and a higher development of biliary, salivary, 

 pancreatic and other glands, to assist in the complex process 

 of assimilation. The digestive canal of the mollusca almost 

 never passes straight through the body, nor is the posterior 

 orifice terminal, as it is in most of the articulata. The gastric 

 cavities are more distinct, more numerous, and capacious; the 

 intestine is more lengthened and convoluted, and the chylo- 

 poietic glands are not only larger, and developed on a higher 

 plan, but are more constant throughout the cyclo-gangliated 

 classes, than in the long extended trunks of the active and car- 

 nivorous worms and insects. The softness or subdivided 

 nature of their food, and the magnitude of their hepatic, 

 salivary, and other glands, enable the molluscous animals to 

 dispense with the numerous solid instruments of mastication 

 and prehension disposed around the mouth in the articulated 

 tribes, and their whole economy being thus adapted for the 

 absorption and solution of the softer and inferior kinds of 

 organized matter, teeth or other dense parts are more rare 

 in their digestive sacs, than in the entomoid and even the 



O * 



helminthoid classes. 



XIV. Tunicata. The tunicated animals, the lowest and 

 simplest of the molluscous classes, subsisting on the minute 

 organic materials suspended in the waters of the sea, and 



