CHAPTER XVI 



PHYLUM MOLLUSCA 



Classes : — i. Gasteropoda, e.g. Snails. 2. Solenogastres — A small 

 class of doubtful worm-like forms, e.g. Neomenia. 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 un- 

 segmented, 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 Arthro- 

 pods are active. In the frequent presence of a trochosphere 

 larva, in the nerve-ring around the gullet, and in some other 

 features. Molluscs resemble Annehds, but it is probable 

 that they took their origin from a still lower level. 



General Characters 



Molluscs are unsegmented and without appendages. The 

 symmetry is fundamentally bilateral, hut 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 dorsal or lateral fold of the body wall forms a mantle, or 

 pallium (Fig. 247, c), which often secretes a single or bilobed 

 shell covering the viscera, and encloses 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 — cere- 

 brals, pedals, and pleurals — with connecting circumoesophageal 

 commissures^ and there is also a visceral nervous system con- 

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