Notes on the Adult Anatomy of Solaster endeca. 181 



1. Serial sections — (a) of very small S. endeca (1*8 to 2 mm. across the 



disc; five examples); and (&) of small S. papposa (16 to 21 mm. 

 across the disc ; three examples). 



2. Examination under the microscope of small cleared or uncleared 



vjhole specimens — (a) S. endeca (2'5 mm.; two examples) ; and (b) 

 small S.papposa (16 to 40 mm. ; four specimens). 



3. Dissection of large specimens — (a) S. endeca, two specimens, one with 



eight and one with nine rays ; and (b) S. papposa, three specimens 

 with ten, thirteen and fifteen rays respectively. 



The result is to show that normally the anus in both species lies in 

 interradius V./VL, the numbering employed being that which I have ex- 

 plained elsewhere (8) to be the natural one in Solaster on developmental 

 grounds. The numbering in question makes the madreporic interradius 

 lie between radii I. and II., the former being on its dextral 1 and the latter on 

 its sinistral 1 side, while the succeeding members of the series, viz., III., IV., 

 V., etc., follow to the sinistral side of II. 



(1) Serial Sections. 



Horizontal series are the best for the purpose, but even they require 

 extremely careful study. 



(a) S. endeca. During the earliest growth stages the anus occupies a 

 comparatively excentric position, and can be located with care by using as 

 guides the most convenient radially disposed structures, viz., the radial 

 cseca of the stomach. 



The rectum at first lies within a mesenteric sheet which anchors it on 

 either side, and which I take to be the remains in this region of the layer 

 which separated the epigastric and the hypogastric customs. In other 

 interradii this layer soon becomes broken up into scattered strands or 

 threads, but here it persists longer, and, indeed, its sinistral wing, i.e., that 

 extending towards ray V., remains permanently in the form of a fibrous fold 

 connecting the rectum with the upper half inch or so of the radial muscular 

 band belonging to ray V. On the other hand, a part of the dextral wing of 

 mesentery on the interradius in question seems to give rise to a comparatively 

 strong and important band of muscular fibres, which in the adult takes origin 

 from the central knot of the aboral musculature, and passes downwards to 

 the junction of rectum and pyloric sac. The disposition of this band suggests 

 that very probably, along with the fold described above, it functions as a 



1 See note at foot of page 174. 



