Contributions to the Anatomy and Histology of Thalassema neptuni Gaertner. 559 



the nerve cord (n. c) and bearing the ventral vessel (Fig. 28 and A m'). 

 Posteriorly it approaches the gut again and attaches itself to the 

 coecuni, on the wall of which the solid continuation of the ventral 

 vessel ends. 



Anteriorly, where it is attached to the oesophagus, it is very 

 difficult to understand, owing to the fact that its ventral attachment 

 is so much shorter than its attachment to the oesophagus. The form 

 which it would present if the dorsal mesentery were cut away and 

 the whole fore-gut brought symmetrically into the sagittal plane, is 

 sketched in Fig. A. Now the relations are complicated by the fact that 

 the oesophageal loop (oe) is firmly bound down to the body wall by 

 a very short dorsal mesentery {d. m) ; the loop then lies to the right, 

 the limb posterior returning dorsal to the anterior one ^). This 

 posterior limb reaches the left side, and then proceeds backwards, 

 still tied down to the ventral body wall, somewhat to the left of the 

 nerve cord, by the dorsal mesentery. 



Owing to the presence of the web of ventral mesentery which 

 occupies the oesophageal loop, the gut, with the two mesenteries, shuts 

 off a portion of the body cavity forwards, forming a kind of false 

 diaphragm. This anterior division of the body cavity opens out by 

 a single small hole (Fig. 28 o) through which the ventral, and at times 

 the dorsal blood vessel passes forwards; this aperture is bounded by 

 the falciform free margin of the ventral mesentery (Fig. 28 m) and by 

 a small piece of the left ventral body wall which lies between the in- 

 sertion of the dorsal mesentery (d. m) and the nerve cord {n. c). This 

 will be made clear by a reference to Fig. 28. 



The radially arranged folds of the peritoneum around the mouth 

 and anus are quite irregular and consequently call for no attention. 



VI. Summary. 



The body wall of Thalassema neptuni consists of a cuticle, which 

 is bound down to the epidermis by a marked alveolar layer; an epi- 

 dermis consisting of at least four different types of elements, viz: typical 

 columnar cells, clear (? glandular) cells, large unicellular glands, and 

 sense organs; the last resemble those of Thalassema diaphanes^ 

 appearing to be simpler than those of Echiurus: a cutis; three raus- 



1) "Anterior" denotes the part nearer to the mouth, "posterior" 

 that nearer to the crop, i. e. further from the mouth. 



