128 IIIROSIII OIIKIIIMA 



III. Hydrocoele absent from both sides. 



[Case 26.] Strongylocentrotus lividus. Runn- 

 strom, 1918 (26), ]). 424, Taf. xiv, fig. 18. Reared at Monaco. 



Amniotic invagination failed to be formed on the left side ; 

 and, instead of it and at the place where the former should in 

 normal case be formed, a calcareous spine appeared. 



[Case 27.] Echinus miliaris. MacBride, 1918 (15), 

 pp. 339-40, PI. vi, figs. 12-14 ; PI. x, figs. 22-3. Reared in 

 London. 



Under this heading more than one specimen will bo described 

 together. Anterior coelom on neither side enlarged so as to 

 form an axial sinus. On nidther side was a hydrocoele 

 discovered, nor was there any vestige of a stone-canal or 

 a dorsal pore. Only in an exceptional case was there found 

 a dorsal pore. In one case a madreporic vesicle was seen and 

 figured (fig. 22). A group of pointed spines developed on each 

 side within the loop of the ciliated band and another spine 

 was found situated dorsal to this loop on both sides. 



G. Considerations on the Organs and Structures 

 concerned and the factors concerned in their 

 Development. 



(a) Anterior Coelom. — This is formed separately on 

 each side pinched off from the posterior coelom, the left one 

 being earlier in its formation than the right fellow (see 

 MacBride, 11, p. 298). Sometimes the two anterior coeloms 

 unite to form a single sac on the dorsal side of the larval 

 oesophagus (Cases 5, 7). The left one is connected with the 

 pore- and stone-canals and remains as a distinct sac, called the 

 axial sinus, while the right one normally remains as a simple 

 sac and very often degenerates later. 



{h) Madreporic Vesicle. — This is a minute round sac 

 normally found a little on the left side lying close to the pore- 

 canal, and often is stated to exhibit a rhythmic pulsation. 

 MacBride (11, p. 299) discovered in Echinus esculentus 

 that this vesicle was derived from the right anterior coelom, 



