MOLLUSC A 207 



of this external opening situated in front of the ctenidium, 

 they are enveloped in a gelatinous coating deposited by the 

 albuminiparous gland, and are impregnated by the spermatozoa 

 of another individual, which has been stored up in the sperma- 

 theca. The spermatozoa when they pass out of the genital 

 pore pass along the spermatic groove, which runs along the 

 right side of the head and terminates in a muscular penis, by 

 the aid of which the spermatic fluid is introduced into the 

 genital pore of another individual and finds its way to the 

 spermatheca. 



The Opisthobranchiata show a considerable tendency to 

 lose some of their organs ; this process of degeneration is 

 carried farthest in one of the sub-orders, the Haplomorpha, in 

 which neither mantle - fold, ctenidia, nor cerata are found. 

 Phyllirhoe has little but its odontophore to show its relationship 

 with the MoUusca. It is a flattened, Planarian-like, transparent 

 pelagic organism, about half an inch long. Its skin contains 

 numerous unicellular glands, which are said to secrete a 

 phosphorescent slime. The ovo-testis in this animal is double. 

 A small Hydromedusa, Mnestra, is frequently found attached 

 by the aboral surface 9f its body to these animals. . In 

 Bhodope the degeneration has gone still farther, and the 

 odontophore has disappeared. 



Eecent research has shown that the group of animals 

 which formerly ranked as a class, the Pteropoda, are really 

 allied to the Opisthobranch Gasteropoda. The Pteropoda 

 are subdivided into two orders, the Thecosomata and the 

 Gymnosomata. The former consists of three recent families, 

 and is allied to the BuUoidea ; the latter contains five 

 families, and is allied to the Aplysioidea, members of the 

 Opisthobranchiata. The failure to recognise the correct 

 affinities of these animals was to some extent due to their 

 external symmetry, but this is a secondary feature which 

 does not affect their internal organs. The animals com- 

 posing this class are all carnivorous and pelagic. In 

 correspondence with their mode of life, they are delicate 

 organisms with transparent tissues, those amongst them pro- 

 vided with a shell — the Thecosomata — having a hyaline one. 

 In both orders the margin of the foot is prolonged and 



