34 MOLLUSCA. 



up into several ganglia, and is connected with the pedal (by the 

 pleuro-pedal commissure) as well as with the cerebral. 



In the Prosobranchiata the position of the visceral commissure, 

 with its ganglia, and nerves presents a peculiar condition (Chiasto- 

 neura) ; the commissure from the right pleural ganglion passes over 

 (dorsal to) the alimentary canal to the left side, and here forms 

 a ganglion the supraintestinal ganglion (fig. 497, G s p] which 

 supplies the left side, while the commissure from the left pleural 

 ganglion passes under (ventral to) the alimentary canal to the right 

 side, and there gives rise to a ganglion, the subintestinal ganglion, 

 which supplies the right side (vide also fig. 510 a). The part of the 

 visceral commissure, which connects the supra- and sub-intestinal 

 ganglia often contains one or more ganglia (V g, A g}. More rarely 

 this crossing is less clearly marked. The cerebral ganglia always 

 give off a pair of nerves, one on each side of the oesophagus, to the 

 buccal ganglia, which give off nerves to the mouth and alimentary 

 canal (fig. 497, B g\ 



Sense organs. Eyes, auditory vesicles (otocysts), tactile and 

 olfactory organs are present. 



The eyes are paired, and are usually placed at the end of stalks, 

 which are as a rule fused with the tentacles. The eyes are largest 

 and most developed in the Heteropoda* in which group they are 

 fastened in special transparent capsules and admit of a movement of 

 the bulb. 



The two otocysts are ciliated internally, and are, except in the 

 Heteropoda, connected with the pedal ganglion (fig. 497, t), 

 although their nerve always arises in the brain. 



Tactile organs are represented by the tentacles, the edges of the 

 lips which are often folded, and lobe-like prolongations which are 

 found here and there on the hend, mantle and foot. There are 

 usually two tentacles ; t exceptionally they are absent (Pterotrachea, 

 etc.). They consist of simple contractile prolongations of the body 

 wall, which can sometimes (Pulinonata) be invaginated into the interior 

 of the body. Certain peculiar hair cells, from which tufts of hairs 

 project in the aquatic Molluscs, are to be looked upon as the seat of 

 a special sensation. They are scattered over the whole surface of 

 the body, and are especially aggregated upon the parts of the body 



* V. Hensen. " Ueber das Auge einiger Cephalophoren." Zeit.fiir miss. Zool., 

 Tom. XV., 18H5. 



f W. Flernminor. " Untersuchungen liber Sinnesepithelien der Mollusken. 

 Arch.fiir. -milt. Anctomle, Tom. VI., 1870. 



