412 



DR ARTHUR T. MASTERMAN ON THE 



./mesenteron 

 Archenteron v'anterior coelom (1 + 2) 



x^coelom 



X 



posterior coelom (2' 4- 3)J 



2' (right body-cavity) 

 3 (left body-cavity) 



Echinoidea. 



For comparison with the Echinoidea we may take the work of MacBride upon 

 Echinus esculentus, a preliminary note upon which has been recently published.* 

 He finds a single anterior ccelomic rudiment from which all the parts of the coelom are 

 derived. This divides into left and right portions. The left is the larger, and on the 

 eighth to ninth day it divides into two parts, the anterior forming the left anterior coelom 

 and hydroccele, united by a constricted stone-canal, whilst the posterior forms the left 

 posterior coelom. 



On the right side the ccelomic vesicle divides somewhat later into right anterior 

 coelom and right posterior coelom. Lastly, at about sixteen days, a small solid out- 

 growth from the right anterior coelom grows dorsally, acquires a cavity, and takes up 

 its position as a closed vesicle dorsal to the oesophagus (fig. 3). 



a. 



b 



d 



5. — The accompanying Diagram is copied from MacBride's work in its outlines, and shows the progressive changes in 

 the right and left ccelomic vesicles. The numbers have, however, been added by myself to indicate the manner ill which 

 the facts described in this work lead me to interpret the changes in Echinus esculentus. 



In this view it is clear that the posterior coelom (3) in Echinus is never dividod whilst the anterior coelom is paired 

 (1 + 1'), the two parts of which are presumably fused at a later stage. 



The position of the central coelom (la) is as in MacBride's figure, but, judging by his fig. 3 and his description, it 

 would be more accurate to draw it further into the median line, when its homology would be even more clear. We may 

 therefore see that Echinus does not differ essentially from Cribrella in the segmentation of its coelom, the chief 

 divergence being the paired origin of the pre-oral ccelom, which is closely paralleled by the pelagic asterids, such as 

 Asterias pallida. 



We may assume that the right and left posterior coeloms are comparable to the 

 similarly named parts of Asterina, but MacBride does not at present indicate the fate 

 of the right anterior ccelom. The small vesicle he compares to the " right hydroccele " 

 of Asterina, and it is evidently homologous to the organ so named by him. If we 

 compare it to the central coelom of Cribrella, we notice that it differs therefrom by its 

 origin from the right-hand side. If the right anterior ccelom is to be regarded as the 



* " The Development, of Echinus esculentus," Proc. Royal Soc, vol. lxix. No. 455. 





