24 ARTHUR E. SHIPLEY. 



mesoblastic skeletal tissue Avhich supports the tentacles of 

 Phoronis as described by Caldwell. The second structure I 

 wish to refer to is the thin membranous fold which I have 

 above termed the collar. This seems to me to correspond 

 very closely with the calyx or web which surrounds the base 

 of the head in Phoronis. 



The absence in the unarmed Gephyrea of mesenteric parti- 

 tions in the post-oral body-cavity^ similar to those which exist 

 in Phoronis, may be accounted for by the twisting of the 

 intestinal loop in the more normal genera. The radial 

 muscles which extend from the visceral loop to the body wall 

 are, in all probability, the remains of an ancestrally continuous 

 mesentery. 



It will be remembered that in Phoronis the body-cavity is 

 divided into an anterior and a posterior division by a trans- 

 verse septum passing from the body wall to the oesophagus, at 

 the level of the nerve-ring. The former division includes the 

 cavity of the praeoral lobe and tentacles, the latter the rest of 

 the body-cavity. I am disposed to think that a similar dispo- 

 sition of parts obtains in Phymosoma. The organ which is 

 usually regarded as forming the blood-vessels in the Gephyrea 

 occupies precisely the same position as the anterior body- 

 cavity in Phoronis; it has, however, acquired a reservoir — 

 the dorsal vessel — into which the fluid may pass when the 

 head is retracted. As this involution is impossible in Pho- 

 ronis no such reservoir has been developed. If this homo- 

 logy holds, there is nothing in the Gephyrea homologous with 

 the true blood system of Phoronis. In connection with this 

 it is perhaps worth noticing that the so-called vascular system 

 in the Gephyrea gives off no vessels or capillaries, but simply 

 consists of a number of intercommunicating spaces. 



April, 1889. The Morphological Laboratory, 



Cambridge. 



