Contributions to the Anatomy and Histology of Thalassema neptuni Gaertner. 557 



IV. Vascular System. 



My observations upon the relations of the blood vessels agree 

 thoroughly with those of Lankester (11) differing considerably from 

 Rietsch's description. Whether the great scarcity of Thalassema at 

 Marseilles prevented Rietsch from gaining a clear insight into the struc- 

 ture of the worm, or whether the examples which he described belonged 

 to another species than Thalassema neptuni^ I cannot say; I am in- 

 clined to the latter view. The chief point in which Rietsch's account 

 difters from Lankester's is that he figures and describes a connexion 

 between the vessels and the peri-intestinal sinus. According to him 

 the two arms of the neuro-intestinal anastomosis do not form a simple 

 ring as in Echiurus, but the three vessels open into a peri-intestinal 

 sinus or "poche v a s c u 1 a i r e" such as he describes in Bonellia minor. 

 Now in the many specimens which I have examined this is never the 

 case. We have always the simple ring , as in Echiurus , such as 

 Lankester describes (fig. 12, 25). Rietsch states that in Bonellia 

 and Thalassema the anastomosis has retreated further backward on 

 the intestine, and that the ring has become converted into a sinus. But 

 in Thalassema neptuni I find that the ring has not retreated backwards 

 on the intestine but occupies the position that Spengel describes 

 and figures in Echiurus, viz: between the crop and intestine (Fig. 12 

 d.v, n.i.a). In Thalassema neptuni the vascular ring is only lightly 

 applied to the crop, and such traces of a sinus as 1 have found occur 

 always on the intestine proper, i. e. behind the pre-intestinal con- 

 striction and consequently behind the blood vascular ring. 



The dorsal vessel may be irregularly dilated at any part of its 

 course, and it is always the thickest of the vessels. It leaves the fore- 

 gut and the mesentery about the level of the gizzard. The relations 

 of the vessels in the proboscis are well known. The ventral vessel 

 is, as Rietsch describes, solid in the posterior part of its course. It 

 runs in the ventral mesentery to the posterior end of the body where 

 it ends blindly on the wall of the coecum. As is well known its walls 

 bear the gonads. The neuro-intestinal anastomosis is the variable 

 part of the vascular system. The two limbs of the "muscle ring" ^) 

 generally open separately into the ventral vessel (Fig. 26 m.r), but 

 sometimes unite first into a single stem (Fig. 27 m. r). The ring 



1) The terminology applied to the vascular system is that proposed 

 by Spengel, 17, p. 509. 



