Mr Doncaster, On rearing the later stages of Echinoid Larvae. 49 



Strong ylocentrotus urchins lived for a month after the meta- 

 morphosis, and during that time the spines increased in length, 

 the skeletal structures became more strongly developed, and five 

 conspicuous Sphaeridia appeared, which were not present at first, 

 but otherwise there was no great change. The hybrid urchins 

 died a few days after the metamorphosis. 



There are two points of some interest connected with these 

 observations. The first is that the difficulty in rearing the later 

 larvae of Echinoids varies very greatly according to the species, so 

 that methods which are successful with one may be of no value 

 when applied to others, and it is therefore important to anyone 

 who proposes to work at the development of this group to choose a 

 suitable species. The second point is that it is possible in certain 

 cases to rear hybrid larvae beyond the metamorphosis, and so to 

 obtain hybrid urchins, and this raises the further question of why 

 such hybrids do not occur in nature. Strongylocentrotus lividus 

 differs very greatly in the adult condition from Echinus micro- 

 tubercidatus, although the very young urchins are very much alike, 

 and it is possible that at the time when these differences would 

 show themselves the young urchins would die off; and as a matter 

 of fact, in the experiments described they only lived for a few 

 days. Or it is possible that the two species normally shed their 

 sexual cells at different times, but in a place like Naples, where 

 both are exceedingly common, and where both are ripe at the 

 same seasons, this by itself seems hardly a sufficient explanation. 



VOL. XII. pt. I. 



