12 EXPERIMENTS IN THE BREEDING OF CERIONS. 



they are often difficult to distinguish. The organ is composed of a 

 large number of minute tubules. 



There are four main muscle bands which follow the columella of the 

 shell far up the interior to their place of attachment ; these, in the order 

 of their attachment, are: the columellar, right cephalic, buccal, and left 

 cephalic muscles. The first spreads out in the tissue of the foot back 

 of the opening to the buccal cavity. The right cephalic in Cerion 

 incanum is attached to the shell with the columellar muscle. At its 

 anterior end it is attached to the vagina and the right side of the head. 

 The right tentacular retractor is attached to it. The right ocular 

 retractor is also attached to it in Cerion crassildbris. The buccal 

 retractor and left cephalic retractor are attached to the shell together 

 and are placed slightly above the other two. The former has been 

 described, the latter is attached to the left side of the head and the left 

 tentacular and ocular retractors are attached to it. One right cephalic 

 muscle in a well-expanded example of Cerion incanum was 12 mm. 

 long. 



While there is a general similarity in the disposition of nerve struc- 

 tures in all of the species examined, there is also some variation. The 

 length of the cerebral commissure is not constant. In Cerion crassi- 

 labris and Cerion viaregis, the cerebral, pleural, and pleuro-pedal com- 

 missures are perfectly distinct, while in the other species the first two 

 elements are superficially attached through a portion of their length, 

 the length of the commissure being so short that the ganglia are adja- 

 cent. The esophageal ring is large and does not bind the esophagus 

 to the body floor, as in some mollusks. In a specimen of Cerion 

 viaregis in which partial contraction had taken place, the buccal mass 

 had been retracted through the ring. The structure of these elements, 

 especially the cerebro-buccal commissure, would indicate that this 

 was the normal state. The cerebral ganglia in normally expanded 

 animals lie on top of the posterior portion of the buccal mass. The 

 heavy connecting commissure passes over the salivary ducts and the 

 esophagus. This is comparatively long in Cerion uva (Linnaeus) and 

 practically absent in Cerion crassilabris. The shape and size of the two 

 side cerebral ganglia is the same. 



There is a forward extension of the optic ganglion (best seen in side 

 views), from which the optic nerve arises. Two nerves arise from the 

 outer side of each ganglion. The upper one is the tentacular, while 

 the lower one passes to the lip of the corresponding side. Both of 

 these, as well as the optic nerve, terminate distally in enlargements 

 which suggest ganglionic development. The cerebro-buccal commissure 

 arises on the lower, inner, and anterior side of the cerebral ganglion. 

 This is not free throughout its length, but becomes firmly bound to the 

 buccal mass before it reaches the buccal ganglion. This condition 

 seems to permit the buccal mass to retract through the nerve ring 



