THE GASTROPODA 161 
fused together for their whole length in other forms. The pedal centres 
are situated at the base of the fin (Fig. 142, v). The visceral commissure 
is fairly long, is crossed, and bears several ganglia, but there is neither 
dialyneury nor zygoneury. In the Carinariidae, however, there are 
secondary uncrossed viscero-pedal anastomoses, and in the Pterotracheidae 
the pedal connectives are fused with the anterior part of the visceral 
commissure, and behind the pedal ganglia the two branches of this com- 
missure are fused together for the greater part of their length. The 
osphradium is a more or less elongated ciliated organ, situated in the 
pallial cavity to the left of the branchia. The otocysts are situated near 
Fic. 141. 
Oxygyrus keraudreni, male, right-side view. A, head; a, mouth and odontophore ; B, anterior 
part of the foot; b, cephalic tentacles ; c, eye; d, natatory foot and its sucker ; e, posterior lobe 
of the foot ; 7, operculum ; i, mantle and pallial cavity ; i, ctenidium ; /:, retractor muscle of 
the foot (columellar muscle) ; 7, optic tubercle ; m, oesophagus ; 7, salivary gland ; 0, rectum and 
anus ; p, liver; g, kidney; s, ventricle ; w, the otocyst attached to the cerebro-pleural ganglion ; 
w, testis; x, auricle of the heart; y, vesicula seminalis; z, penis. (From Lankester, after 
Souleyet.) 
the cerebral ganglia (Fig. 141, wu). The eyes are very large and highly 
differentiated in structure ; they are placed at the sides of the cerebral 
ganglia and at the bases of the tentacles (Fig. 141, c) when the latter organs 
exist (Pterotrachea and the female in some Firoloida are devoid of tentacles). 
The alimentary canal is furnished with a protractile pharynx containing a 
characteristic Taenioglossate radula with very powerful lateral and marginal 
teeth. The oesophagus is very long and slightly dilated in the middle of 
its length. The stomach and liver are situated posteriorly (Fig. 142, m) , 
the intestine is always very short, and in the Pterotracheidae it is no longer 
bent forward (Fig. 143). /The heart is situated near the stomach, and in 
the less specialised Heteropoda (Atlantidae, Carinariidae) is clearly disposed 
in the same manner as in other Streptoneura, but in the Pterotracheidae, 
which have undergone detorsion, it has clearly become an opisthobranch 
ia 
