50 Hjalmar Théel, 



remains of -the pluteus skeleton. Besides, sea-urchins with such re- 

 mains are by no ixieans rarely met with in the open sea. In the year 

 1846 Müller ') gave some figures of a sea-urchin with larval remains 

 and recently Fewkes ^) has figured a young Echinarachnius bearing long 

 rods on the dorsal surface. Considering that these remains are in 

 want of a cellular investment and that on the whole only scattered 

 fragments of tissues are left on them, PL VIII, fig. 102., it seems 

 not possible that they can be absorbed by the wandering cells or 

 phagocytes as evidently is the case with the greater part of the lar- 

 val skeleton. They probablj'^ are simply broken and thrown off from 

 the body. 



After this short survey of the changes which take place during 

 the growth of the young sea-urchin within the Pluteus, I may be allowed 

 to describe the stages of young Echinocyamus which I have succeeded 

 in raising in my aquaria. 



The very young sea-urchin, which has just lost most of the 

 larval appendages but still retains some calcareous rods protruding from 

 the dorsal surface, presents itself as a minute sphere or oval with the 

 feet, spines and spherids encircling it in such a manner as to form a 

 ring or boundary between the greater dorsal region and the small ventral 

 one, PL VIII, fig. 101 — 102. The young one is now about forty-five days 

 old and has a diameter of about 0,24 mm. It wants oral as well as anal 

 openings. A closer examination of it offers a good deal of interest. 

 The ventral space, PL VIII, fig. 106., in the centre of which the mouth 

 eventually breaks open in older stages, is smooth, pigmented and pre- 

 sents a more or less obviously pentagonal form. It has become inclo- 

 sed with a ring of fifteen delicate perforated plates, which gradually 

 began to appear in the earlier stages, when the sea-urchin was still 

 retained within the Pluteus. The arrangement of these plates in the 

 young Echinocyamus agrees remarkably well with that in young spe- 

 cimens of Abatus cavernosus, described and figured by Loven ^) in his 

 beautiful work on Pourtalesia. Here as well as there, the ambulacra are 

 indicated by five pairs of calcified plates and the interambulacra by five 



1) Ueber die Larven und die Metamorphose der Ophiuren und Seeigel. 1846. 

 pi. VII. 



2) Preliminary Observations on the develoj^ment of Ophiopholis and Echi- 

 narachnius. 1886. pi. YIII. 



3) On Pourtalesia, a genus of Echinoidea. K. Sv. Vetenskaps-Akademiens 

 Handlingar. Vol. XIX. N:o 7. Stockholm 1883. p. 25. pi. 14. 



