226 DISCOVERY REPORTS 



the antennolabral apodeme through the isthmus and into the thickness of the shell 

 where it divides at once into two main branches, one running dorsally and the other 

 ventrally. The dorsal branch gives off a group of very small nerves which form a plexus 

 among the strands of the adductor muscle. These join up by a single nerve with the 

 more posterior shell nerve III. 



Behind the shell nerve II is a group of five nerves which may be called the mandibular 

 group. Of these, one makes its exit definitely on the ventral side of the nerve ring and 

 is the main origin of the labral visceral system (Fig. n). This will be dealt with later. 

 Another nerve, not so ventral in origin as the last, and at the posterior margin of the 

 group, runs directly into the mandible where it enters the large mandibular basal 

 ganglion. It may be termed the mandibular sensory nerve. Of the remaining three 

 nerves two are relatively small and run direct to the musculature of the mandible. They 

 can be traced with certainty to their termination on the muscles of the mandible. They 

 constitute the anterior and posterior mandibular motor nerves. The last member of 

 the group is large and makes its exit more dorsally than the remainder. Soon after 

 leaving the nerve ring it divides into two separate branches (Figs, n, 14). Of these 

 the lower is the larger branch and is the shell nerve III. Like the second of this series 

 it divides into dorsally and ventrally running branches which ramify over the surface 

 of the valves. The upper branch is the nerve to the adductor muscle. I have traced it 

 on to the final nerve endings on the separate strands of the adductor muscle (Fig. 14). 



The maxillulary group of nerves behind the mandibular illustrates well the variation 

 that occurs among the origins of these nerves. In Fig. 13 it will be seen that on one 

 side the group consists of three distinct nerves, while on the other there are only two. 

 This type of variation was found throughout the nerve ring but more particularly at 

 the hinder end. The most dorsal member of this group (Fig. 1 1) is really a double nerve 

 consisting of an anterior cardiac nerve together with a shell nerve IV. I shall deal with 

 this nerve later. The remainder of this group supplies the maxillule. 



The main maxillary roots are two in number on either side. One, which represents 

 a mixed nerve, in addition to supplying the main maxillary muscles, supplies the 

 musculature of the vibratory plate and arises mid-way between the maxillulary group 

 and the long trunk which marks the beginning of the ventral chain. The second arises 

 close alongside this trunk and is a sensory branch swelling up almost immediately into 

 a large basal ganglion (Plate XL, fig. 7). There is a very small nerve which arises from 

 the dorsal side of the trunk just before it swells up into the terminal swelling of the 

 chain (Fig. 11). This supplies a large muscle which attaches anteriorly to the front edge 

 of the maxilla and posteriorly to the adductor muscle. I described this nerve in Doloria, 

 but in this form it is as large as the other two maxillary nerves. 



The nerves to the first trunk limb arise from the ventral side of the stalks of the 

 ventral chain immediately below the small third maxillary nerves (Figs, n, 13). They 

 enlarge almost at once to form the basal ganglion of the limb. 



Behind the nerves to the first trunk limb the nerves become very irregular in their 

 origin (Fig. 13). The next following nerve is connected with the heart and dorsal 



