CHAPTER XIII 



PHYLUM MOLLUSCA 



MOLLUSCA (Lat. mollis, soft) is the name which is given to one 



of the largest and most important phyla of the animal kingdom. 



In it are included not only our terrestrial snails and slugs and 



many fresh-water species but also the oysters, mussels, periwinkles, 



whelks and countless other species of "shell-fish," bivalve and 



univalve, which crowd the rocks laid bare at low- 



General water around our coasts : and in addition to these, 



description. 



the extraordinary Octopuses, Squids and other forms 

 of Cuttle-fish belong to the same great phylum. The name Mollusca 

 seems to have been suggested by the fact that the members of the 

 phylum do not possess any internal hard parts such as are found in 

 Man and other vertebrates. This softness of internal constitution 

 is shared by other classes with no relation to the Mollusca, as for 

 instance the great group of the Arthropoda. The Arthropods how- 

 ever possess a horny covering which closely invests them and follows 

 every irregularity of their outlines, so that it seems a real part of 

 themselves. This is the exoskeleton or cuticle, which constitutes one 

 of the great differences between them and the Mollusca. The latter, 

 it is true, possess also an exoskeleton composed principally of cal- 

 careous matter, but this adheres only to a part of the surface. It is 

 usually very thick and easily detached, and so it is frequently looked 

 on as a separate thing from the animal and is known as the shell. 

 The shell is to be looked on as a secretion produced by a part of 

 the skin only : this part of the skin, which almost always projects 

 from the rest of the body as a flap, is called the mantle. The 

 space between the mantle-flap and the rest of the body is known as 

 the mantle-cavity. The mantle-cavity shelters the gills or organs 

 of respiration, and into it open the kidney or kidneys and the anus, 

 and usually also the genital ducts. 



Class I, GASTROPODA. 



In order to fix our ideas we may take the common English garden 



Description snail, Helix aspersa, which has also established itself 



of snail. throughout considerable areas in North America, or, if 



procurable, the larger Helix pomatia, which on account of its size 



