22 Animal Life by the Sea-shore. 



III.— MOLLUSCS. 



The animals with which we shall deal in this chapter are 

 known as shellfish, in the more restricted sense of the terra, slugs, 

 cuttlefish and their ahies, and constitute the division MoUusca, 

 one of the most important of the animal kingdom. This division 

 is well circumscribed, and the bonds of union between its very 

 numerous members are apparent enough, notwithstanding the 

 fact that, adapted for most diverse modes of hfe both on land 

 and m water, their structure is often profoundly mochfied in 

 accordance. Three organs are especially characteristic of the 

 Mollusc, and are present in the more typical examples, viz., 

 the foot, the mantle and the shell. The foot is the organ of 

 locomotion, essentialty a thickening of the ventral musculature ; 

 it often attains a very great relative size, and can be modified 

 to subserve almost any kind of movement — creeping, swimming, 

 grasping or burrowing ; it is very rarely absent, as in a few 

 fixed forms, the Molluscs, like most other groups of animals, 

 including strongly degraded types. The mantle is a fold of 

 the dorsal integument, enclosing a space known as the mantle- 

 cavity, the primary function of which is the protection of the 

 breathing organs, of the nature of giUs in the tj'pical marine 

 forms. The mantle, moreover, secretes tiie shell, an inorganic, 

 calcified structure which protects or affords complete shelter to 

 the soft body. The shell is, however, much reduced or absent 

 in many fonns. 



The Molluscs are divided into four primary groups, the 

 names of which are deri\'ed from the structure of the loot : 

 (i) Cephalopods (Octopus, Cuttlefish and Squids) ; (2) Gastro- 

 pods (Snails, Slugs and Cluton) ; (3) Pelecj^pods (Bivalves) ; 

 (4) Scaphopods (Tooth-shells). In the first two of these groups 

 there is normally a distinct head, bearing sense-organs in the 

 form of eyes and tentacles ; in the other two the head is so 

 much reduced as to be almost unrecognisable. 



The Cephalopods are the most highly organised of the 

 Molluscs, and in many ways contrast strikingly with the other 

 members of this great division. They have a better brain, 

 protected by a cartilage, and their large e^'es recall those of 



