GENERAL CONSIDERATIONS. 257 



may unite directly or become closely associated by means of unusually heavy 

 connectives. At frequent intervals the pedal cords are united by commissures, 

 while .an approximately siinilar number of connectives unite them with the 

 lateral ganglia. These last named elements are united posteriorly by a heavy 

 suprarectal commissure. The labiobuccal connectives pass to ganglia in 

 the neighborhood of the radula, which are united by a ventral commissure. 

 A dorsal commissure has also been demonstrated in certain species of neomenians 

 (for example, Dorymenia acuta and Strophomenia scandens) arising from the 

 labiobuccal ganglia or on the labiobuccal connectives. A second ventral 

 commissure may also exist. In a few species of both suborders a subradular 

 system has been demonstrated with ganglia, commissure and connective unit- 

 ing them with the labiobuccal gangUa. 



With these general features in mind the innervation of the various regions 

 of the body will now be described, the description being based solely upon data 

 derived from the study of species I have personally examined. In the Chaeto- 

 dermatina the nerves from the great precerebral ganglionic masses attached 

 to the anterior surface of the brain innervate the frontal sense organ and the 

 anterior end of the digestive tract. Nerves from the lateral cords, and in some 

 instances from the latero-pedal connectives, attach to the somatic musculature 

 dorsally and laterally, while fibres from the pedal cords pass to the more ventral 

 portions of the body. In the posterior end of the animal the ventral section 

 of the cloacal wall and the adjacent region of the body wall are supplied with 

 nerves originating at the posterior end of the united lateral and pedal cords. 

 The ventral gill retractor and the ventral half of each gill is supplied with a 

 nerve from the ventral side of the suprarectal commissure, while the dorsal 

 half of the gill is penetrated by a nerve arising close to the mid line on the dorsal 

 side of the suprarectal commissure. Another pair of nerves, originating on the 

 dorsal side of this same commissure but more laterally situated, spread over 

 the dorsal cloacal wall and the neighboring portions of the body wall probably 

 including the dorso-terminal groove. In Chactoderma hawaiiense a nerve from 

 the suprarectal commissure, in the mid Une, has been followed into the tissue 

 surrounding the rectum. Nerves from the labiobuccal ganglia extend pos- 

 teriorly along the wall of the fore gut, and in Chaetoderma eruditum have been 

 followed as far as the stomach. The subradular organ is supplied with nerves 

 from the subradular ganglia. 



In the Neomeniina the six nerves leaving the anterior surface of the brain 

 are in part distributed to gangUa, about the bases of the cirri, from which deli- 



