188 Proceedings of the Royal Physical Society. 



the axial sinus, while it had not yet made its appearance on the body wall to 

 the dextral side of this sinus. 



The haemal tissue of the genital sinuses is directly continuous with that of 

 the aboral sinus, and ends by spreading out in branches on the inner of the 

 two layers of which the gonad walls are composed. 



Of the two great gastric hasmal tufts one, as shown in Fig. 8, is connected 

 with the hasmal tissue of the aboral sinus on the same side, and the other with 

 the correspondingly situated haemal tissue on the opposite side. The axial 

 organ is continuous above with the tissue invaginating the dorsal sac, and at 

 the sides with the haemal tissue of the aboral sinus, and thus with the genital 

 and gastric extensions. The lumen of the aboral sinus is continued for a 

 short distance into the interior of the root of each of the gastric tufts, but 

 particularly into that of the sinistral one. 



5. An Interbrachial Branch of the External Oral Circular 



Sinus (Fig. 8). 



While working at the later development of Solaster, I noted that in each 

 interradius, from about the first month after metamorphosis onwards, there 

 could be seen a slight widening of the external oral perihaemal cavity, and 

 that this widening was traceable aborally into a short stem which became 

 lost by dividing up into branches in the thickened connective tissue of the 

 interradial body wall. 



After verifying this condition in several specimens of similar age, I 

 examined carefully some series which I had made of portions of a very large 

 adult Solaster, and these enabled me to study the question in four interradii, 

 one of which was the madreporic interradius. In all four I was able to 

 follow out a slender but quite definite branch which, arising from the external 

 oral perihaemal sinus, passes aborally between the ring canal of the hydrocoele 

 and the odontophore, and gaining the inner side of the interbrachial septum 

 continues for a short distance embedded in the connective tissues underneath 

 the coelomic lining. In the latter part of its course it gives off numerous 

 slit-like branches and then it disappears, no cavity derived from it being 

 traceable in my sections higher up than about a quarter of the whole height 

 of the interbrachial septum. 



A Polian vesicle is present in all the interradii examined, except in the 

 madreporic one. It arises by a long slender stem in the mid-interradial 

 line from the ring canal of the hydrocoele, and passes in an aboral direction 

 at first through the tissues at the base of the interbrachial septum. The 

 perihsemal branch just described lies close outside and parallel to this part 



