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 bronchia. The otocysts are situated near 



FIG. 141. 



Oxygyrus kcraudrnni, male, right-side view. A, head ; , mouth and odontophore ; B, anterior 

 part of the foot ; h, cephalic tentacles ; c, eye ; </, natatory foot and its sucker ; c, posterior lobe 

 of the foot ; /, operculum ; h, mantle and pallial cavity ; i, ctenidium ; A;, retractor muscle .of 

 the foot (columellar muscle) ; /, optic tubercle ; m, oesophagus ; ., salivary gland ; o, rectum and 

 anus ; p, liver ; /, kidney ; s, ventricle ; n, the otocyst attached to the cerebro-pleural ganglion ; 

 w, testis ; x, auricle of the heart ; y, vesicula scminalis ; z, penis. (From Lankester, after 

 Souleyet.) 



the cerebral ganglia (Fig. 141, u). 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 orgai* 

 exist (Pterotrachea and the female in some Firoloida are devoid of tentacles). 

 The alimentary canal is furnished with a protractile pharyn* 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, w) , 

 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 



ii 



