202 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 
stout 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- 
clion, 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 their contents leave the body through the 
right nephridium. 
Order 2. AZYGOBRANCHIATA. 
CHARACTERISTICS.— The <Azygobranchiata, which constitute the 
other order of the Streptoneura, have lost their original left 
ctenidium and osphradium. The right nephridium does not 
exist as such, but is most probably represented by the generative 
