566 THE INVERTEBRATA 



single kidney which is not used for the passage of the genital products. 

 The gonads have separate ducts and in the male there is a penis. 



The eggs are laid in capsules which usually contain several hundred 

 and the capsules are attached to each other, forming the sponge-like 

 masses so often flung up by the tide. 



Murex is nearly related to Buccinum and also carnivorous. It has 

 been recently shown that the salivary glands and the "liver" all 

 contain the same proteolytic enzymes. These have been separated 

 by adsorption and found to comprise a proteinase, a carboxy-poly- 

 peptidase, an aminopolypeptidase and a dipeptidase. These are just 

 such enzymes as occur in the vertebrates and the higher Crustacea, 

 but in contrast to vertebrates in Murex there is no division of labour 

 amongst the digestive organs. 



Fig. 388. Pterotrachea. co.g. cerebral ganglion; cr. crop; ct. ctenidium; 

 e. eye; /. foot (fin); ped.g. pedal ganglion; su. sucker; t. teeth which prevent 

 large particles of food from passing before digestion ; vis.g. visceral ganglion ; 

 vis.h. visceral hump. 



LittorinUy the periwinkle, is interesting because it exhibits tend- 

 encies toward a terrestrial habit which is reflected in its structure. In 

 certain species the filaments of the ctenidium are extended over the 

 roof of the mantle cavity to form a kind of vascular network not un- 

 like that in Helix and other pulmonates. Littorina rudis lives almost 

 at highwater mark and spends more of its life in air than in water. 



The structure of this form is very similar to Buccinum but it has 

 no proboscis and is not carnivorous. 



Paludina, on the other hand, is a freshwater form of common 

 occurrence in this country which still preserves the ctenidium and so 

 must be regarded as a direct immigrant from sea water into fresh 

 water. It possesses a kind of uterus in which embryos of relatively 

 enormous size are developed. 



Pterotrachea (Heteropoda) (Fig. 388) is an inhabitant of the open 

 sea with many adaptations to pelagic life. It is laterally compressed ; 

 the tissues are transparent except for the digestive gland and peri- 



