CHAPTER VII 

 MOLLUSCA 



I. GASTEROPODA Snails, Slugs, Whelks. 

 II. SIPHONOPODA (or Cephalopoda) Cuttlefish, Squids. 

 III. PELECYPODA (or Lamellibranchiata) Mussels, Oysters, Clams. 



THERE can hardly be a contrast more striking than that between the 

 typical Arthropods, with their hard, jointed bodies and their active move- 

 ments, and the typical Molluscs or Shellfish, with their soft fleshy bodies 

 and extremely sluggish movements and yet as will be seen later there 

 are certain fundamental features of structure in which the two groups 

 are alike. 



In examining the external appearance of the mollusc one is at once 

 struck by the absence of two conspicuous arthropodan features the 

 segmentation of the body and the possession of paired appendages. It 

 is only in two of the most ancient types of mollusc, represented respect- 

 ively by Chiton the most ancient existing type of Gasteropod, and 

 Nautilus the most ancient existing type of Siphonopod, that we find 

 traces of segmentation present hinting to us that the far back ancestral 

 creatures from which the molluscs of to-day are descended had in all 

 probability segmented bodies. 



The mollusc possesses typically a definite head region at the tip of 

 which is the mouth and which carries a pair of eyes. The Gasteropod 

 (Fig. in, A) has usually also one or two pairs of tentacles, the epidermis 

 covering which is crowded with sensory cells. In the Siphonopod the 

 head region extends into numerous sticky tentacles (Nautilus Fig. 112), 

 or into a circle of eight or ten long arms (Fig. in, B) carrying numerous 

 powerful suckers by which the animal grasps its prey or, when at rest, 

 holds on to its rocky substratum. 



The mollusc like the arthropod possesses a hard exoskeleton, but 

 this instead of covering the entire surface is confined to the skin of 



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