182 Proceedings of the Royal Physical Society. 



sphincter separating the rectum from the pyloric sac, besides aiding in the 

 evacuation of feces from the rectum. The band becomes evident a 

 surprisingly short time (less than four weeks) after the anal opening has 

 formed. While one is naturally chary of attempting to explain growth 

 changes on mechanical grounds, it is open to suggest that the constant 

 traction which this band is capable of exercising may have something 

 to do with the final approximation of the anus to the centre of the 

 aboral disc. 



Of the five tiny Solaster endeca examined by the method of serial sections, 

 one did not give convincing evidence, three showed the anus to be in V. to 

 VI., while the other was a noteworthy exception inasmuch as the anus lay in 

 interradius VI./VII., and there was a group of five (instead of four) roots of 

 radial caeca associated together in that division of the disc which lies between 

 the madreporic and the anal interradii counting sinistrally from the former. 

 This is the only aberrant instance which I have come across by any of the 

 methods. 



(b) Serial sections of small S. papposa (16 to 21 mm.; three examples). 

 Here also the sections were horizontal. The series, however, were far from 

 perfect, owing to the difficulty of cutting the tough aboral body wall. By the 

 time the size above indicated is reached, the anus has moved relatively nearer 

 the centre of the disc. On this account, and also because a vastly greater 

 number of sections, some of which were partly broken up, had to be studied, 

 I could not have relied with certainty on these series for fixing the anal 

 interradius, but in all three it was clear that the anus lay at least in the 

 close neighbourhood of interradius V./VL, and they were therefore of use 

 in confirming the result of the other lines of evidence. 



(2) Examination under the Microscope of small whole Specimens 

 cleared or uncleared. 



(a) S. endeca. (2 to 5 mm.; four examples). Cleared in clove oil, all these 

 specimens showed more or less distinctly the position of the anus and its 

 relation to the origins of the radial (and also, in the case of the oldest, of the 

 rectal) c< r eca. One of the specimens was remarkable in having only eight rays. 



(b) S. papposa (16 to 40 mm.; four examples). Clearing with clove oil 

 ^ave a similar but less satisfactory result, the anus being now nearer the 

 centre of the disc, and therefore more difficult to locate. Two of the 

 specimens were large enough to allow the anus and the madreporite to be 

 seen externally even before clearing was resorted to. One of the specimens 

 had eleven rays and the others thirteen. 



