66 
Proceedings of Royal Society of Edinburgh. [sess. 
efferent vessel being partially moved back from its true asym- 
metrical position. 
These two asymmetrical features are probably connected the one 
with the other. I leave out the nephridia in order not to compli- 
cate matters. 
In the region of the stomach the two blood-vessels communicate 
round the gut by a sinus formed between it and the coelomic 
epithelium. 
In diagram 2 the separation is partially effected so that the 
lateral mesenteries are seen to be composed of the dorsal mesentery 
a. 
of the anterior half of the trunk and the dorsal peritoneum of the 
posterior half. 
As the posterior part of the trunk moves out it allows of the 
approximation of the two anterior elements of the lateral mesentery 
to approach one another in the median line, and the same applies 
to the two nerve cords. 
In diagram 3 is depicted the effect produced by a complete 
separation of the two trunk halves, (a) Is a transverse section 
through the anterior half, and ( b ) an inverted transverse section of 
the posterior half. 
This section (a) I would compare directly to one of Balanoglossus. 
