COLLAR-CANAL MUSCLE OF CEPHALODISCUS. 41 



coelomie corpuscles (co. co.}. At the extreme basal end of the stolon a median 

 septum is seen extending from the blood-vessel v. 2 to that marked v. 3. It is 

 continuous with the body septum that stretches between the pharynx and the 

 ventral body wall and divides the trunk cavity into a right and a left com- 

 partment, but it does not extend far down the stolon, and is already incomplete at 

 the level of section represented in text-fig. 15. 



Lying in the mesenteric fold below the stomach is a blood-vessel (v. 1) which 

 passes forward, and on reaching the base of the stolon, passes downward, then 

 backward along the dorsal wall of the stolon, and comes again into view in the 

 section now under consideration at v. 2. The blood-vessel marked i' 3 is continued 

 forward into the body, running between the pharynx and the ventral body wall. 



Passing longitudinally below the stomach, at mu. 7 in text-fig. 15, are some 

 muscle fibres which can be traced forward as far as the base of the stolon. Here they 

 curve down and then run back along the dorsal side of the stolon, by the side of the 

 dorsal vessel of that organ (v. 2). The muscle fibres in the lateral and ventral parts 

 of the stolon (mu. 8) are continued forward into the trunk as an extensive tract lying 

 ventrally and laterally to the pharynx. The ventral part passes straight forward 

 and stops a little distance behind the mouth (text-fig. 10, mu. 2), against the septum 

 between the collar cavity and the trunk cavity. The lateral part, which is more 

 bulky, forks on reaching the gill-slit into one part which runs dorsally to the slit 

 and collar canal (text-figs. 13 and 14, mu. 5) and gradually thins out as its fibres are 

 inserted into the body wall, and a second part which runs ventrally to the gill-slit 

 and collar canal and ends at the front of the right and left anterior horns of the 

 trunk cavity (text-fig. 14, mu. 6). 



Lying immediately internal to the superficial epithelium of the stolon is a thin 

 layer of muscle fibres, circularly disposed. In transverse sections of the stolon this 

 layer might easily be taken for a layer of skeletal tissue ; but in tangential sections 

 of the layer the outlines of the individual fibres can readily be seen. The fibres 

 are much finer than those of the bulky longitudinal muscles of the stolon, and are 

 arranged in a very even and continuous sheet. 



Collar-canal Muscle (Problematical Tissue). 



The transverse section of the collar canal of C. nigrescens has the form of an 

 elongated ellipse set obliquely. The epithelium of the postero-dorsal side is composed 

 of tall, closely-set prismatic cells bearing long cilia, the epithelium of the antero- 

 ventral wall is thin, and is in relation with a short, stout tract of muscle fibres, 

 which pass obliquely from this antero-ventral wall of the collar canal and the part 

 of the body-wall which lies to the antero-ventral side of the collar pore. The most 

 ventral of these fibres are attached to the dorsal (aboral) surface of the lateral 

 flap of the post-oral lamella, the rest have no obvious attachment. This muscle 



