MOLLUSCA 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 Mollusca. 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 of its body to these animals. In 
Rhodope the degeneration has gone still farther, and the 
odontophore has disappeared. 
Recent 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 Bulloidea; 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 
