CHAPTER XVI. 



MOLLUSCA. 



Classes I. AMPHINE0RA — A small class of bilaterally symmetrical 

 forms, e.g., Chiton. 2. Gasteropoda, e.g.. Snails. 3. Scapho- 

 PODA — A small class, of which the best known is Dentalium. 

 4. Lamellibranchiata — Bivalves. 5. Cephalopoda— Cuttle- 

 fishes. 



The series of Molluscs stands in marked contrast to that of 

 Arthropods, for the body of the Mollusc is unsegmented, 

 and there are no appendages. The general habit of life is 

 also very different, for, although there are active Molluscs 

 and sluggish Arthropods, it is true as an average statement 

 that Molluscs are sluggish and Arthropods active. Though 

 the pedigree is unknown, there does not seem to be any 

 possible ancestry for Molluscs less remote than the stock 

 from which Turbellarians and other unsegmented " worms " 

 have sprung. 



General Characters. — Molluscs are unsegmented and 

 without appendages. The symmetry is fundamentally 

 bilateral, but this is lost in most Gasteropods. The "foot" 

 — a muscular protricsion of the ventral surface, is very 

 characteristic ; it usually serves for locomotion, but is much 

 modified according to habit. Typically, a projecting dorsal 

 fold of the body wall for7ns a mantle, or pallium {Fig. no, <:.), 

 which often secretes a single or bilobed shell covering the 

 viscera ; but both mantle and shell may be absent. There are 

 three chief pairs of ganglia — cerebrals, pedals, and pleurals, 

 with connecting commissures, and often with accessory ganglia, 

 especially two viscerals on a loop connecting the pleurals 

 {Figs, no, n8). Except in Latnellibranchs, in which the 

 head region is degenerate, there is in the mouth a chitinotis 



