Contributions to the Anatomy and Histology of Thalassema neptuni Gaertner. 553 



to a part of the original groove, but is, in my opinion, a new structure 

 formed by the union of the two furrows (f) which lie outside the 

 two lips of the original ciliated groove. This groove (formed by the 

 fusion of these furrows), which in the middle of its course becomes 

 more pronounced, I shall refer to under the name of "secondary ciliated 

 groove". Fig. 23 represents sections from a series cut in this region 

 and explains itself, it being obvious here that the collateral intestine 

 owes its existence to the fusion of the summits of the two longitudinal 

 ridges or lips that bound the ciliated groove. 



The hinder end of the collateral intestine (Fig, 13) on super- 

 ficial examination resembles the anterior one. When, however, the 

 dorsal wall of the intestine is dissected away and the specimen 

 examined, as described above, in oil of cloves, we find a slight dif- 

 ference, viz: the ciliated groove runs forward for a short distance 

 (1 — 2 mm) in front of the circular opening (Fig. 22 c.gr') by which 

 the "Nebendarm" opens into the gut, and there ends by the union 

 of its walls. This slight difference will at once be recognised on 

 reference to the sketches. 



This difference indicates, so far as I can see, merely a more 

 gradual transition from the groove to the siphon posteriorly than 

 anteriorly. In all the examples that I examined in this way these 

 conditions were constant at both ends ; there is always posteriorly 

 this short horse-shoe-shaped process of the ciliated groove. 



At the posterior end also the nature of the secondary groove is 

 plainly visible (Fig. 22 s.gr). I can find no cilia in the collateral 

 intestine except just at the two openings where the condition is 

 transitional from groove to siphon. So far as I can see the groove 

 is generally stronger and deeper posterior to the collateral intestine 

 than anterior to it. The groove ends in the position described and figured 

 by RiETSCH, i. e. to the right of the opening of the little coecum on 

 the beginning of the rectum. 



In contrast to the fore-gut, as already stated, the intestine pre- 

 sents an external circular and an internal longitudinal layer of 

 muscles. The layer of circular muscles is seen in longitudinal sections 

 at the pre-intestinal constriction to be continuous from crop to in- 

 testine. On the other hand I can find no trace of continuity between 

 the two layers of longitudinal fibres, and am led to regard the inner 

 longitudinal musculature of the intestine as a structure sui generis. 

 I will return to this point later on. The musculature never seems 



36* 



