EEPOET ON THE ECHINOIDEA. 179 



from the large size and the opacity of the egg and embryo it is not a very favourable 

 species for observation, had other conditions been favourable we had all the material for 

 working out the earlier stages in the development of the young very fully. The eggs, on 

 being first placed in the pouches, are spherical granular masses of a deep orange colour, 

 enclosed within a pliable vitelline membrane, which they entirely fill. They become 

 rapidly paler in colour by the development of the blastoderm ; they then increase in size 

 probably by the imbibition of water into the gastrula cavity, and a whitish spot with a 

 slightly raised border indicates an opening which I have no reason to doubt is the per- 

 manent mouth, but of this I cannot be absolutely certain. 



" The surface now assumes a translucent appearance, and becomes deeply tinged with 

 dark purple and greenish pigment, and almost immediately, without any definite inter- 

 mediate steps, the outer wall is filled with calcified tissue ; it becomes covered with fine 

 spines and pedicellarise, a row of tentacular feet come into action round the mouth, the 

 vent appears at the posterior extremity of the body, and the young assumes nearly the 

 form of the adult. These later changes take place very quickly, but they are accompanied 

 by the production of so much heavy purple and dark green pigment that it is difficult to 

 follow them. The viscera are produced at the expense of the abundant yelk, and the 

 animals at once take a great start in size by the imbibition of water into the perivisceral 

 cavity. The young urchins jostle one another on the floor of the breeding pouch, those 

 below pushing the others up until the upper set are forced out between the rows of fringing 

 spines of the pouch, but even before leaving the marsu^^ium, on carefully opening the shell 

 of the young, the intestine may be seen already full of dark sand, following much the same 

 course which it follows in the adult. The size of the test of the young on leaving the 

 marsupium is about 2*5 mm. in length by 2 mm. in width." 



To give as fully as possible the history of this species the accompanying description of 

 the changes due to growth are reprinted from a notice on the Viviparous Echinids, from 

 Kerguelen Island,^ describing the early stages of this species. 



"The function of the deej^ly sunken petaloid ambulacra of several genera of 

 Spatangoids, such as Moira, Schizaster, Hemiaster and the like, has thus far remained 

 unknown. Philippi in 1845, while describing some South American Spatangoids, found 

 in the deeply sunken posterior ambulacra of Hemiaster cavernosus minute Echinids, which 

 he regarded as the young of the species, though they differed widely from the adults, and 

 seemed, from their shape and the nature of their spines, to approach nearer the regular 

 Echinids than the Spatangoids. Echinids of this genus being but rarely found in 

 collections, no opportunity occurred of verifying the observations of Philippi. A some- 

 what analogous observation was made by Grulie, who described more in detail the 

 young of Anochanus {Echinobrissus), which he found living under very similar 

 circumstances, in a cavity opening in the abactinal pole of the specimens. No details of 



' A. Agassiz, 1876, Proc. Am. Acad., p. 231. 



