i6o 



THE GASTROPODA 



hermophrodite with separate male and female gonads ; parasitic in the 

 testis of Holothurians, causing their abortion. Three species are 

 known : one in Synapta digitata (Mediterranean), one in Holothuria 

 edulis (Philippines), and one in a Holothuria from Puget Sound in the 

 North-East Pacific. Enteroxenos, Bonnevie ; no pseudopallium and no 

 alimentary tract ; male and female gonads separate, with a single 

 common genital orifice ; larvae operculiferous. E. ostergreni (Fig. 



1 40) ; parasitic in the intestine of 

 Stichopus (Norway). 



FIG. 139. 



Kntomticha mirabilis, in situ, mag- 

 nified. I, oral extremity ; II, remains 

 of the digestive tract; III, testis; IV, 

 ovary; V, antimesenteric vessel of 

 the Synapta in which Entoconcha is 

 parasite. (After J. Miiller.) 



FIG. 140. 



Enteruxenos ostergreni, Bonnevie. 

 of, eggs. (After Bonnevie.) 



TRIBE 2. HETEROPODA. 



These are free - swimming Taenioglossa, with the foot flattened 

 laterally and the otocysts situated near the cerebral ganglia. There 

 are no mandibles and the intestine is short. All the Heteropoda are 

 pelagic, and are much modified in adaptation to this mode of existence. 

 The foot is very large, and has the form of a fin compressed bilaterally ; 

 it bears, in the male at least, a sucker on its ventral aspect (Fig. 1 42, d'). 

 The visceral sac or " nucleus " and mantle form a progressively smaller 

 and smaller part of the mass of the body (compare Figs. 142 and 143), 

 but the head always remains large and forms a cylindrical snout. The 

 cerebral nerve-centres are in juxtaposition; the pleural ganglia, still 

 visible in the Atlantidae and Pterotracheidae, are attached to them, and 

 there are thus two pedal connectives on either side, namely, the cerebro- 

 pedal and the pleuro-pedal ; these are separate proximally in Atlanta, but 



