SVXAPTA VIVIPARA. 75 



ring. No trace of mesenchymatous musculature was found anywhere, and the part 

 which the meaoderm takes in the formation of the haemal system is certainly incon- 

 siderable. 



8. THE ANATOMY OF THE ADULT. 



Although the anatomy of the European Synaptas is so well known, thanks to the 

 investigations of Baur ('64), SiMium ('87), Hamann ('83 and '89), Cuenot ('91), and 

 others, there are so many points in which Synapta vivipara differs from the forms 

 hitherto examined, it seems desirable to add a few words concerning these and other 

 points. Except in the case of sense-organs, no attempt has been made to go into the 

 histology, but my attention has been confined to the more general features of the minute 

 anatomy. In the structure of the body-wall and the muscular system, there are no 

 important features to mention, aside from the shape of the longitudinal radial muscle 

 bands. Each of these bands is forked at its anterior extremity, and the two branches 

 are attached to the radial calcareous plate, one on each side of the radial nerve. These 

 branches soon unite as they pass backward, and form a single narrow band, which 

 extends far out into the body-cavity. But still further back, it decreases in depth and 

 increases correspondingly in width, and the epithelium which covers it tends to fuse at 

 the outer edges with the epithelium of the body-cavity, so that at numerous points in its 

 course the muscle has acquired secondary attachments to the body-wall. During the 

 greater part of its course, it is a nearly flat band, but as it approaches the extreme 

 posterior region of the body, it tends to become cylindrical, and where it ends near the 

 anus the cross-section is circular. These changes in shape will be made clear from Figs. 

 94-100. The structure of the genital glands has already been given in detail, and the 

 openings in the wall of the rectum have also been sufficiently described. The blood- 

 vascular or haemal system is very simple, consisting of a dorsal and ventral vessel on the 

 intestine and stomach with connecting lacunae in their walls. Posteriorly, both vessels 

 end about half way down that section of the intestine which lies in the right ventral 

 interradius (Fig. 92). Anteriorly, the ventral vessel ends a little in front of the stomach, 

 on the oesophagus. The dorsal vessel runs forward to the water-ring and forms on its 

 inner side a circumoesophageal ring, from which branches pass on to each tentacular 

 vessel. The dorsal blood-vessel also seems to open out in the mesentery to form broad 

 lacunae about the genital gland, such as Cuenot ('91) found in European Synaptas, but I 

 never found coagulated blood there as in the dorsal vessel, and I do not feel sure that 



