37 



ctenidium and nephridium on the right side of the body did 

 not atrophy, but are as large, and sometimes larger, than 

 those on the left side; and the Azygobranchiata, in which 

 the ctenidium and nephridium on the right side of the body 

 have been lost. This last is a very large group, including 

 the majority of the Gastropoda, and is considerably more 

 modified than the Zygobranchiata. 



The Heteropoda are a divergent group of the Strepto- 

 neura. They agree with the Azygobranchiata in their 

 essential characters, but have become adapted to a free- 

 swimming existence. The foot is especially modified in 

 some forms {e.g., Carinaria) into a flattened fin-like organ, 

 provided with a sucker. The visceral mass may become 

 greatly reduced, and the shell may be entirely lost. 



The Euthyneura are characterised by their straight vis- 

 ceral nerves, and by the atrophy of the paired organs on the 

 right side of the body. They are all hermaphrodite, and in 

 many cases the shell is absent. The ancestral Euthyneura 

 are represented by the Opisthobranchiata of the present 

 time, including two series, the Nudibranchiata and the 

 Tectibranchiata ; while the Pulmonata are a divergent group, 

 derived from the primitive Opisthobranchiata. In the Opis- 

 thobranchiata the heart is placed anteriorly to the base of 

 the gill. In the Nudibranchiata the mantle-skirt and the 

 shell are both absent. Some extraordinarily modified forms 

 belong to this latter group. In many the ctenidium is 

 absent, and respiration is performed by other processes from 

 the body wall. 



The Pulmonata have become adapted to a terrestrial life. 

 The ctenidium has been lost, and the paUial cavity has 

 become converted into a respiratory sac, communicating 

 with the exterior by a small aperture. In some forms 

 {LimaXy &c., the slugs) the visceral mass is greatly reduced, 

 and the shell is either very small or altogether absent. 



