56 Hjalmar Théel, 



a small dilatation, fig. 112. Subsequently the end of the tooth grows 

 more powerful and commences to give off spines more or less ob- 

 tuse, which, when the tooth has reached such a length that it becomes 

 visible through the just opened mouth, which happens at about sixty- 

 days after the fecundation, PL IX, fig. 109., appear not only on the 

 lateral sides, but on the cutting edge, where they grow large and 

 blunt. This is a fact of interest, since in the fully mature Echinocj^a- 

 mus the teeth are devoid of spines. As to the carina, there are scarcely 

 yet any traces of it to be detected. 



The question whether the teeth originate by the agency of cells 

 folded in from the epithelium or from migratory cells, may at present 

 be left undecided. However, I believe that the buccal membrane cannot 

 partake in this process, and on the whole I was never able to observe 

 any kind of invagination of ectoderm or entoderm. Giesbrecht ') has 

 in an excellent manner described the structure of the teeth in the sea- 

 urchins and I hope in the future, when I have succeeded in raising 

 older stages of Echinocyamus or of other sea-urchins, to be able to 

 confirm his statements, the specimens hitherto at my disposal having 

 been too young. 



After this short sketch of the metamorphosis of the sea-urchin, I 

 may perhaps describe in a few words the oldest young one I have raised 

 in my aquaria, which reached an age of about two months, PL IX, fig. 

 107. It has assumed an oval form, and its dorsal surface is convex 

 while the lower one is almost plane. The measurements are as follows: 



Length of the body without spiues .... 0,4 mm. 



Breadth of the body » » .... 0,26 mm. 



Length of the pointed spiues 0,2 mm. 



Length of the two posterior pointed spines . 0,3 mm. 



Length of the truncated spines 0,ii mm. 



If we glance at the figure, the bilateral arrangement must 

 appear very striking. Not only do the spines in general seem to be 

 symmetrical in position, but especially the two long posterior spines 

 contribute to give the animal a bilaterally symmetrical aspect. Besides, 

 considering the smallness of the spines in the adult animal, it must be 

 surprising to find them in the young attain such a length. The buccal 

 membrane has just become pierced, and the mouth thus produced pre- 



1) Der feinere Bau der Seeigelzähne. — Morphol. Jahrbuch. VL Leipzig 

 1880. p. 79—105. 



