THE GASTROPODA 



161 



fused together for their whole length iu 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 



nuuo , j/j 11VC1 , '/, JYIUUCJ , dj VOllblxUIQ , tt, LUG ULUU^ > L rtl/UtUUCU tu LUG CCICUIU-JJlolliaj Jfail^llun . 



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



the cerebral ganglia (Fig. 141, ). 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 mai-ginal 

 teeth. The oesophagus is very long and slightly dilated in the middle of 

 its length. The stomach and liver are situated posteriorly (Fig. 142, ri) , 

 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 



