The anatomy of Lottia gigantea Gray. 39 



buccal mass but to the side of the head cavity, is a thin sheet of 

 white muscle and connective tissue, often interrupted or divided into 

 several bands, segregating the cerebro-pleural and cerebro-pedal con- 

 nectives into side pouches of the head cavity. In Fig. 13, showing the 

 dorsal view of the buccal cartilages, a muscle (23) is shown, which 

 anchors the lateral cartilages to the main radula support. Another 

 muscle (24) passes between the two halves of the support, and binds 

 them closely. The tissue between these two halves is rather soft and 

 not rigid enough to form a firm connection. 



Nervous System. 



Cerebral ganglia and nerves arising therefrom (Fig. 33). 

 The cerebral ganglia are situated on either side of the buccal mass 

 at the base of the tentacles, and are connected one with the other 

 by a long ribbon-like commissure, which embraces the front of the 

 buccal mass. Each ganglion, as well as the cerebro-pleural and 

 cerebro-pedal commissures, is situated in a sort of incomplete side 

 chamber of the head sinus, formed by thin dorso- ventral sheets of 

 muscle (18 Fig. 3). From each ganglion the following commissures 

 and nerves arise : the cerebral commissure, labial commissure, cerebro- 

 pleural and cerebro-pedal commissures, two prominent nerves to the 

 lips (ul, ul% a strong tentacle nerve (in), a very slender nerve to 

 the eye (op), several minute nerves to the base of the tentacle (tn% 

 a fine nerve to the otocyst and two small nerves to the dorsal wall 

 of the head cavity (hw). 



The cerebral commissure lies perfectly free in the head cavity, 

 but is held in place to a certain extent by slender muscle bands which 

 anchor the buccal mass to the front wall of the head sinus (Fig. 3). 

 From the upper edge of the commissure three or four, probably more, 

 very fine nerves are given off to the head wall. Below the cerebral 

 commissure, and from the anterior ventral portion of each ganglion 

 two important nerves (ul, uV) pass downward and forward into the 

 musculature of the lips, particularly the portion in front. These two 

 nerves soon anastomose and also send a branch or two backward to 

 join nerves from the labial ganglia. As shown by the figure (Fig. 34) 

 the system not only supplies the lips but also, by dorsal branches, 

 the whole front of the head. 



From the outer face of each ganglion three sets of nerves pass 

 into the tentacle of that side. Counting from the front the first, as 

 already mentioned, is the tentacle nerve (in). This soon divides into 



