ZOOLOGY 



From the posterior end of each cerebral ganglion two com- 

 missures pass backward, the outer one passing into the pleural 

 ganglion, the inner to the pedal. Each pleural ganglion is 

 connected with the pedal of its own side, and the two pedals 

 are united by a pedal commissure. The pleural gives off two 

 stovit nerves. The outer of these soon splits, one branch going 

 to the gills and mantle, the other to the circular muscle which 

 attaches the animal to its shell. The second nerve given 

 off from the pleural forms the origin of the visceral loop. This 

 is a nervous loop, which, starting at each end from the pleural 

 ganglion, forms a figure of 8 twist. In its course it gives off 

 a nerve to an olfactory ganglion lying at the base Of each of 

 the rudimentary ctenidia. The olfactory nerve going to the left 

 ctenidium arises from the loop near to the right pleural gan- 

 glion, that to the right ctenidium arises near the left ganglion. 

 This twisting of the visceral loop is characteristic of the 

 Streptoneura. 



The pedal ganglia give off each two large nerves, which 

 supply the muscles of the foot. 



The tentacles have a tactile function; at their base the 

 eyes are situated — they consist of a pair of pits sunk in the 

 surrounding tissue. The epidermal cells lining these pits 

 become modified and deeply pigmented, and are connected by 

 an optic nerve with the cerebral ganglia. A similar simple 

 eye, consisting of an open pit lined with pigmented cells, is 

 found in Nautilus. 



Limpets are dioecious ; the position of the generative glands 

 is similar in the two sexes (Fig. 118) between the muscular 

 foot and the digestive organs, rather near the posterior end. 

 Like other members of the Zygobranchiata, the generative glands 

 possess no ducts, and theircontents leave the body through the 

 right nephridium. 



Order 2. Azygobeanchiata. 



Characteristics. — The Aeygdbranchiata, which constitute the 

 other order of the StreptoTieura, have lost their original left 

 ctenidium and osphradium. TJie right nephridium does not 

 exist as such, hut is most prohably represented iy the generative 



