222 



ZOOLOGY. 



by oviparous generation; it happens therefore frequently 

 that individuals springing from each other remain united 

 together, constituting animated masses, phytoid, in fact 

 (Pig. 180). 



The molluscoides are subdivided into two classes, according 

 as they have the respiratory apparatus enclosed in the mouth, 

 or formed by a corona or circle of long labial tentacula. 

 The first are called tunicata (Fig. 180) ; the second forms the 

 class bryozoa or polyzoa. 



The mollusca, properly so called, differ amongst themselves 

 by characters whose importance is still very considerable. 

 Thus, in some the cephalic ganglions are very distant from 

 the abdominal ; there is no distinct head, nor trace of the 

 special organs of the senses ; the organs of movement are ex- 

 tremely imperfect, and the body is 

 wrapped up by cutaneous folds, pro- 

 tected exteriorly by a bivalve shell 

 (Fig. 183). Muscles, oysters, &c., 

 present this mode of organization. 

 Other mollusca, as the snail, limnseus 

 (Fig. 166), and the cowry (Fig. 

 181), have a distinct head : their 

 nervous ganglions are generally close 

 to each other, and grouped around 

 the gullet ; they have eyes ; the 

 lower surface of the body is occu- 

 pied by a fleshy organ, serving for 

 locomotion; finally, the back is 

 generally protected by a shell, and 

 this is never bivalve, but represents 

 almost always a cone turned into a spiral (Fig. 182) ; others 

 have a distinct head like the preceding^ and on each side of 

 the neck a kind of membranous wing, which serves as an oar 

 (Fig. 184). 



Finally, there are some which have the head furnished 

 with long contractile and prehensile appendages, performing 

 the functions of feet and arms (Fig. 185). 



These have the nervous system more developed than in 

 other animals of the same primary division, and which gene- 

 rally have the body supported by a sort of interior shell. 

 Such are the various modes of conformation serving as a 

 basis of the division of the mollusca, properly so called, into 

 four classes, called acephala, gasteropoda, pteropoda, and 



Fig. 184. Hyalsea. 



