MUSEUM OF COMPARATIVE ZOOLOGY. 139 



through the wall of the pluteus. The line of the edge of the future 

 Echinarachuius cau be traced on the dorsal side of the stomach of the 

 gluteus, while the " dorsal pore," ma, lies near the edge of the dish 

 turned in the direction of the oral region of the pluteus. The growth 

 of this opening is a migration from the vicinity of the posterior arm 

 towards the middle line of the dorsal side of the body. As it grows in 

 this direction it works at the same time to the anal apex of the pluteus, 

 never, however, reaching that position. Unlike the figure of a Spatan- 

 goid pluteus, PL VIII. fig. 13, by Metschnikoff, the retort-shaped vesi- 

 cle before division into the rosette does not extend so that the dorsal 

 pore lies in the median line. In the figures which we have of the 

 young Spatangoid, the line bounding the wall of the growing Clypeas- 

 troid is always recognized on the dorsal surftice of the body in stages as 

 old as fig. 8, PI. VIII., of Metschnikoff 's paper on the development of 

 Echinoderms. I have given a series of figures to illustrate the relative 

 changes in position of the dorsal pore, ma, from very early conditions, up 

 to a stage when the deposit of pigments renders observation impossible, 

 lu the progress of this migration of the madreporic body or dorsal pore 

 it will be observed that the length of the ambulacral tubes increases, 

 and additional feet form as diverticula, while interesting calcareous 

 deposits occur, PI. VII. fig. 9. It was not observed whether these feet 

 bud from the five primary tubes or not. There is no reason to doubt 

 that they do. The appearance of pigment spots on the body of the 

 forming sea-urchin takes place at about the same time as that of the 

 tritid rods which they later obscure. The first limestone formation 

 which was observed is a trifid spicule in the wall of the body of the 

 growing sea-urchin. In its very first form this trifi^d spicule is spheri- 

 cal in contour. Later, it assumes a trifid shape, and seems to be en- 

 closed in a transparent sac, the outer wall of which is believed to be 

 formed of epiblast, the calcareous body being formed possibly in meso- 

 blast. This transparent sac and its enclosed calcareous body of tripod 

 shape resembles the structures, cc, in E. lividus, as figured by Metsch- 

 nikoff.* If these bodies are morphologically the same in Schizaster 

 and Echinarachuius, we have a likeness hitherto unrecorded between 

 the young Spatangoid and the immature Clypeastroid. 



Metschnikoff figures, PI. VI. (fig. 10, op. cit.,) in the Ophiuran plu- 

 teus a similar calcareous body, to which he gives the name of " Hohl- 

 kehlen," already used by Miiller. 



Ludwig has already remarked on the resemblance of similar cal- 



* Op. cit., PI. VIII. fig. 9. 



