492 THE INVERTEBRATA 



and highly developed proboscis which can be retracted within a 

 proboscis sheath. The true mouth is situated at the end of the pro- 

 boscis. The radula (of the rachiglossate type), is used for rasping 

 away flesh, but it can even bore holes in the carapace of Crustacea. 



There is only a single ctenidium with a single row of filaments. 

 This is the primitive right member of the pair, though situated on 

 the left of the mantle cavity. A very prominent organ is the bipectinate 

 osphradium, which is easily mistaken for a ctenidium. There is a 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. 



Littorina, the periwinkle, is interesting because it exhibits tendencies 

 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 unlike that 

 in Helix and other pulmonates. Littorina rudis lives almost at high- 

 water 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 not 

 a proboscis and is not carnivorous. The radula is taenioglossate in 

 type. 



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) 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 pericardium com- 

 pressed into a small visceral hump. The animal swims ventral surface 

 uppermost, using its foot as a fin. The sucker is a rudiment of the 

 crawling surface. It is predaceous, seizing worms and other animals 

 with its radula and swallowing them whole. 



Order OPI STHOBRANCH I ATA 



Hermaphrodite gasteropods which are descended from Streptoneura 

 which have undergone torsion but themselves show a reversal of 

 torsion (detorsion) ; with the mantle cavity, where present, tending to 

 occupy a posterior position again, the shell to become smaller, in- 

 ternal or entirely absent and the single ctenidium to disappear and 

 be replaced by accessory respiratory organs or by the whole external 

 surface becoming a respiratory organ. 



