21 



forma". We have thus e. g. Ophiopluteus undulatus nov. forma, species a, 

 species b. etc. This method will, I think, meet all reasonable claims 

 to a system of larval nomenclature, which will remain a necessity so long 

 as we do not know the origin of all the larvae we meet with in the 

 plankton. 



For the description of the larvae it was felt a necessity to have a con- 

 sistent nomenclature of the parts of the larval body, and such was worked 

 out in the author's said work in the Plankton Expedition, which nomen- 

 clature has also been generally adopted by the recent workers, excepting 

 most of those dealing with the larvae only from an experimental point 

 of view. In the said work it was maintained to be less appropriate to 

 designate the larval prominences as "arms", which could not, in any case, 

 be used in the descriptions of Auriculariae and several forms of Bipinna- 

 riae, and it was accordingly advocated that they should be termed "pro- 

 cesses". This has not been generally adopted. I agree it is not a necessity 

 either and, since it is really somewhat troublesome with all those "pro- 

 cesses" in the descriptions — the more so as it cannot be avoided speaking 

 of "processes" also in the skeletal parts — I have given it up and am 

 designating the long prominences of the larval body as arms, while the 

 short, less differentiated prominences of the larval body in forms like 

 Auricularia may be termed processes or lobes. This is easier and con- 

 fusion is not likely to occur for that reason. 



In the following descriptive part the Echinoids are placed first, then 

 the Ophiuroids, Asteroids and Holothurians. This is due to the fact that 

 the Echinoids have yielded the best results, then the Ophiuroids etc. 

 No other meaning is connected with this arrangement. That no observ- 

 ations on Crinoids are included in the present work is due to the fact 

 that my studies on the development of Crinoids have been published 

 in a separate work^). 



I. Echinoidea. 



Of all Echinoderms the sea-urchins are the most favourable objects 

 for artificial f ertiUzation ; indeed, there is hardly any other group in the 

 whole animal kingdom which equals them in this regard. If specimens 

 with well ripe genital products are available, one may nearly always 

 be sure to obtain fertilization and to secure good cultures. Even in such 

 cases, where the eggs are large and yolk-laden I have never had any 

 difficulty in obtaining fertiUzation in the usual way, while in the case 



1) studies in the development of Crinoids. Papers from the department of Marine 

 Biology of the Carnegie Institution, Washington. Vol. XVI. 1920. 



