CHAPTER XII 



MARINE MOLLUSCS 



THE sub-kingdom Mollusca includes a great variety of soft-bodied 

 animals which differ from the members of the last division in the 

 fact that they are never segmented, and in the possession of a thick 

 outer covering, of a leathery nature, which completely envelops the 

 body, and which usually secretes a calcareous shell of one or more 

 parts. A general idea of the extent of the group may be formed 

 when we state that it contains the Octopus and the Cuttlefish; 

 all Snails and Slugs, and animals of a similar nature ; and all those 

 numerous ' bivalves ' which are represented by the well-known 

 Oysters, Mussels, Scallops, &c. 



By far the greater number of the molluscs are aquatic in habit ; 

 and of these such a large proportion are marine that the group 

 provides plenty of occupation for the sea-side naturalist. This 

 being the case, we shall devote the present chapter to a description 

 of the general characteristics of these animals, and to the principles 

 of their classification, illustrating our remarks by a few selections 

 from all the chief divisions. 



Although, as we have already hinted, the body of a mollusc 

 generally bears but little resemblance to that of the typical elon- 

 gated and segmented worm, yet the study of the earliest stages of 

 the former shows that a certain relationship exists between the 

 two sub-kingdoms, the newly hatched mollusc being often a minute 

 free-swimming creature with expanded lobes fringed with cilia, 

 and bearing a resemblance to certain of the Rotifers, Moss Polyps, 

 and other annuals that are included among the Vermes. But in 

 the adult molluscs this resemblance is lost, these creatures being 

 generally easily distinguished from all others by certain well- 

 marked external features, as well as by internal characters that 

 are peculiar to them and fairly constant throughout the group. 



The external shell, where it exists, is usually composed of one 

 or of two parts, and therefore we speak of univalve and bivalve 



