23 



tion was undertaken repeatedly, none of the cultures proved very suc- 

 cessful, evidently on account of the water being not of sufficiently good 

 quality. The youngest stages were excellent, but as soon as the formation 

 of the Pluteus was about to begin, the larvae became abnormal and died. 



The eggs are very small — I regret to have omitted to notice the exact 

 size — and very transparent. The developing embryos are most beautiful 

 objects, the cells are large and very clear, and the cell limits are very 

 distinct. 6 hours after fertilization they were swimming blastulse; about 

 20 hours after fertilization the gastrula stage was reached. 



The young larva is a very curious object, rather unlike other young 

 Echinoderm larvae. It has the shape of a biconvex lens, with a perfectly 

 circular band, and recalls, in fact, a Trochophora by its shape (PI. V 

 Fig. 1), while it is, of course, quite different from a Trochophora in its 

 anatomical structure. The digestive organs are very small, the mouth 

 opening being on the upper, the anal opening on the lower side. 



This stage was reached in three different cultures, and there is no 

 reason to doubt that this is the normal shape of the young larva. It was 

 only after this stage was reached that the cultures went wrong. Only 

 in one case I succeeded in getting them a little farther in their develop- 

 ment. In PI. V Fig. 2 is figured a young Pluteus which is, to all appear- 

 ance, normal. It is no less peculiar than the younger stage. The postoral 

 arms are quite horizontal. (In the specimen drawn, — the only good 

 specimen available — the left arm is slightly upwards directed. It 

 has been corrected in the figure to suit the right arm, which is quite 

 horizontal and has all the appearance of having preserved its natural 

 form and direction, while the other has more the appearance of being 

 slightly distorted. However, since the skeleton is dissolved, it is impos- 

 sible to ascertain definitely which is the correct form of the arms — and 

 I have omitted to notice anything about it from the living specimens.) 

 Other processes are not yet indicated; there is only an elevation indicating 

 the oral and anal lobe. This larva is 6 days oM. The skeleton was begin- 

 ning to form at the age of three days. Unfortunately the skeleton has 

 been dissolved in all the presei-ved specimens, and I have omitted to 

 make drawings of the living specimens. It was only noticed that the 

 postoral rods are fenestrated. 



In spite of the incompleteness of this record of the development of 

 Eucidaris Thouarsi it is sufficient to show that it differs most conspicu- 

 ously from Cidaris cidaris (Dorocidaris papillata), the only Cidarid the 

 development of which was studied hitherto^). The young larva of this 



1) H. Prouho. Recherches sur le Dorocidaris papillata et quelques autres ficliinides de 

 la M6diterran6e. 1888. Arch. Zool. expdr. gdner. 2. S^r. V. PI. XXIII— XXV. 



Dr. Racovitza has kindly sent me a pair of specimens determined by Prouho as Doro- 



