CHAPTER XVI. 



MOLLUSCA. 



Classes I. Amphineura — A small class of bilaterally symmetrical 

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

 poda — A small class, e.g. Dentalium. 4. Lamellibranchiata — 

 Bivalves. 5. Cephalopoda — Cuttle fishes. 



The series of Molluscs is in many ways contrasted with that 

 of Arthropods ; thus 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 are active. The 

 pedigree is unknown, but 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 protrusion 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 forms a mantle, or 

 pallium (Fig. 148, c), which often secretes a single or bilobed 

 shell covering the viscera, and roofs in a space — the mantle cavity 

 —within which lie the gills. 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. 148, 152). Except in Lamelli- 



