ECHINODERMA. 5 



duct. There is little doubt that this genital base corresponds to the genital stolon 

 of other Echinoderms. In a transverse section through the region of the tentacles 

 there is to be seen in each of the other four interradii a mass of deeply staining 

 rounded cells (PI. II., fig. 8a). The masses are attached to the body-wall, and 

 their component cells in some cases show a tendency to group themselves round a 

 lumen. Is it possible that these are vestigial antimeres of the genital organs 

 destined to be absorbed ? Our material does not allow us to answer this question, 

 as it shows neither the origin nor the fate of these structures. In the body-wall 

 on the right hand side of the section may be seen sections of rudimentary tube- 

 feet. The tip of the tube-foot, that is, the disc of sensory epithelium, appears as 

 an invaginated cup (pod. ect., figs. 5-8, PI. II.) quite distinct from the outgrowth 

 from the radial-canal, which forms the inner part of the organ (pod. end., figs. 5-8). 

 From the radial nerve-cord proceeds a pedal nerve, accompanied, like the nerve 

 from which it took its origin, by an epineural space. 



On each side of the mouth sections of the nerve ring are seen, outside of 

 which are sections of the epineural ring (ep.), but there is no perihsemal ring. The 

 buccal tentacle on the right side is seen to be retracted into a pocket of the oral 

 disc (or.); on the left side a section of one of the five valves of the oral vestibule 

 is seen. 



As we examine similar sections through older specimens the same features can 

 be made out, and the alimentary canal gradually acquires the characteristic 

 curvature into three loops so well marked in the adult. The stomach remains 

 straight and the main portion of this curvature is due to the lengthening of the 

 intestine. In the latest stage which I found in the collections, a thin-walled 

 outgrowth of the rectum on one side (pul., fig. 7) may be regarded as the rudiment 

 of the pulmonary trees and a duct connects the axial sinus with the coelom. This 

 is the beginning of the "secondary madreporite" so characteristic of the adult (mad., 

 fig. 6). 



If we turn to transverse sections we see in fig. 8a the ten buccal tentacles 

 surrounding the mouth. In fig. 8b, a section taken lower down, we can see sections 

 through the five radii. In each we observe a section of the nerve cord, external to 

 which is the epineural canal. Internal to it we find a space lined by a thin flat 

 epithelium, the radial perihsemal canal. Internal to this is the radial water vascular 

 canal, and this is present in each radius, although only in the three ventral are sections 

 of tube-feet recognisable. In the adult Psolus, according to Professor Ludwig (3), the 

 two dorsal radial canals are absent. When the radial canals are followed in successive 

 sections to their tips they do not, as in Echinoidea, Asteroidea and Ophiuroidea, end 

 in terminal azygous tentacles, but terminate within the body-wall. 



The most distal tentacles are laterally placed outgrowths of the median of the 

 three radial canals which bear tube-feet. These are shown in fig. 8c, which repre- 

 sents a section from the same series taken near the posterior end of the body. 



