Contributions to the Anatomy and Histology of Thalassema neptuni Gaertner. 555 



The terminal anus lies in a depression surrounded by the swollen 

 papillae on the hinder end of the body. The walls of the rectum are 

 thicker than those of the intestine, and more muscular. This division 

 is characterized further by the presence of the ventral mesentery 

 which extends as far forwards as the coecum. The free margin of the 

 mesentery bearing the blind end of the ventral vessel, ends on the 

 surface of the coecum. 



Around the rectum are a number of irregular radial mesenteric 

 folds or frenula. The course of the rectum is from the coecum straight 

 backwards. The nerve cord ends with a slight swelling under the 

 rectum, and its primitively double nature is here more clearly visible 

 than elsewhere. It seems to send nerves into the muscular coats of 

 the rectum. On the hinder part of the rectum the musculature is 

 very much confused, while more anteriorly it is arranged as in the in- 

 testinal region, though somewhat thicker. 



In the posterior portion of the rectum the epithelium is strongly 

 ciliated, i. e. from the point where the anal vesicles open into it, to the 

 anus. This division is somewhat dilated and is surrounded by the 

 curious peri-anal glandular organ discovered by Rietsch, which, lying 

 for the most part outside the musculature, I regard as a derivative 

 of the glandular tissue of the skin papillae, the glands opening through 

 the musculature into the rectum. It does not extend further forward 

 than the openings of the anal vesicles. This division behind the 

 openings of the anal vesicles has been aptly termed cloaca by 

 Rietsch. 



When a comparison is made between the digestive system of 

 Thalassema neptuni and that of the other Echiuroids we find a great 

 resemblance. In Echiurus, according to Spengel's description (17), 

 the only difference in the anterior division is that the oesophagus 

 and gizzard cannot be distinguished from one another. In Bonellia 

 minor ^ according to Rietsch's description (12), there can be little 

 doubt that the "intestin buccal" is the equivalent of the fore-gut in 

 TJialassema neptuni. The chief différence here is that the vascular 

 system embraces the alimentary canal further back ; in fact, it embraces 

 the division of the intestine that bears the siphon, not, as in Thalassema 

 neptuni, the pre -intestinal constriction. The vascular system, ac- 

 cording to Rietsch, enters into communication with the peri-intestinal 

 sinus or "poche vasculaire" in Bonellia, and he describes the same 

 condition in Thalassema neptuni, but, as will be seen when I come 

 to the description of the vascular system, his account differs in many 



