558 



H. lA^STEll JAMESON, 



around the alimentary canal may also show variations e. g. in the 

 example figured (Fig. 26), where the right half of the ring is absent, 

 the vessel of the left side alone being present. Fig. 27 shows another 

 variation in which the right limb of the ring is apparently not developed 

 but in which a union occurs between the left vessel and the dorsal 

 vessel on the right side. This can best be explained if we 

 imagine that the right half of the ring is fused for a part of its length 

 to the dorsal vessel. 



Amoeboid corpuscles occur in these vessels, chiefly in the neigh- 

 bourhood of the ring. The spherical corpuscles of the body cavity 

 are not found in the vascular system. 



V. The 3Ieseiiteries. 



We can recognise in Thalassema neptuni a dorsal mesentery, and 

 a ventral mesentery, and a varying number of irregular folds of the 

 peritoneum anteriorly and posteriorly. 



The dorsal mesentery (Fig. 12 d.m) is attached to the alimentary 

 canal throughout its entire length from mouth to anus, being, so far 

 as I can see, only unrepresented in the region of the gizzard. It is 

 always inserted on the dorsal side of the intestinal canal, but its 

 attachments to the wall of the body are irregularly disposed, sometimes 

 dorsal, sometimes lateral, and sometimes ventral. On examining preserved 

 specimens it always appears more or less broken and discontinuous 

 at its insertions on the body wall, but is is possible that this is not 

 so in the living worm. 



The ventral mesentery 

 (Fig. 28 V. m, Fig. A in text 

 v.m), bearing the ventral 

 vessel throughout its entire 

 length, is attached to the 

 alimentary canal anteriorly 

 and posteriorly, that is to 

 the pharynx and oesophagus 

 and to the rectum. At the 

 hinder end of the oesophagus 

 it suddenly leaves the di- 

 gestive tube with a falciform 

 free margin (Fig. 28 m, text-figure m) and extends as a short free 

 fold along the ventral surface, attached to the peritoneal sheath of 



Fig. A. 



