18 DR. TH. MORTENSEN. 



doliolum.] Ein Beitrag zur Keimblattertheorie," Z. w. Zool. XVII, 1876). 

 The difficulty liere is to get the eggs in condition for fertihzation. 



The Auricularia of H. nigra very much resembles that of H. tubulosa, 

 as figured by Selenka (o^:*. cit., Taf, X, 9). The colour is the same as 

 shown there, only I find it more equally distributed in the vibratile 

 chord; also there are some patches of this colour over the body, and 

 especially in the posterior end. Unfortunately I did not make a drawing 

 from a living specimen ; the few preserved specimens are not so well 

 preserved that it would be worth giving a figure therefrom, and the 

 spicule has been dissolved. Still, the information given here will be quite 

 sufficient for recognizing the larva, especially in the waters off Plymouth, 

 where no other Holothuria species occurs with which it could be con- 

 founded. 



Judging from the larva, H. nigra must be nearly related to H. luhulosa. 

 The suggestion made in my Echinodermenlarven der Plankton Expedition 

 (p. 15), that the larva of H. tubulosa will prove to have a spicule at the 

 posterior end in later stages, is very much supported by the fact that 

 such a spicule is found in the larva of H. nigra. A similar calcareous 

 spicule is also found in Auricularia stelligera (Joh. Miiller's " Auri- 

 cularia mit Kugeln"); but it has besides some pink elastic spheres, of which 

 there is no indication in the larvae of H. nigra and tubulosa, so that the 

 A. stelligera would not seem to be very closely related to these larvae. 



