132 



floating apparatus, as appears to be the case in the Crinoids Tropiometra 

 carinata and Antedon petasus.^) 



Eminently characteristic of this larva is the body skeleton. The body 

 rods are simple, nearly horizontally directed; there is a deep sinuation at 

 the point of issue of the rods of the arms. The end rods and the transverse 

 rods are very short and broad, forming together, as it were, a link between 

 the two parts of the skeleton. — The stage figured is too young to show 

 the shape of the fully formed larva; it is only evident that the postero- 

 lateral arms are fairly broad. By means of the skeletal characters it is, 

 however, possible with certainty to recognize some of the larvae in my 

 material as belonging to the genus Ophiocoma, and through these it is 

 again proved that the larva described in my Echinodermen-Larven d. 

 Plankton-Exped. (Taf. VII, Fig. 2) under the name of Ophiopluteus Henseni 

 is the larva of Ophiocoma which is, accordingly, peculiar through having 

 a kind of epaulets or rather vibratile lobes, otherwise unknown in Ophiurid- 

 larvse, excepting only the larva of Ophiocomina nigra. 



The type specimen of Ophiopluteus Henseni had the skeleton dissolved. 

 There is thus no possibility of ascertaining whether the latter is the fully 

 developed larva of Ophiocoma echinata; 1 would rather be inclined to think 

 that it could not be that species, the shape of the young larva difTering 

 not inconsiderably from the 0. Henseni; especially the hind edge of the 

 body is so different that it could hardly depend on age (or preservation) 

 alone. In conformity with the nomenclature adopted in this work, I shall 

 then designate the Ophiopluteus Henseni as Ophiopluteus of Ophiocoma, 

 species a, referring to the description and figure given in the work quoted. 



Species b. The shape of the body appears to agree very completely 

 with that of species a; none of the specimens in hand are, however, suf- 

 ficiently well preserved for being figured. The skeleton (Fig. 58) differs 

 from that of the 0. echinata-lavva in the body rods being quite horizontal; 

 also the basal part of the posterolateral rods is horizontal, the result being 

 an extraordinary width of the skeleton, corresponding to the width of the 

 posterior part of the larval body, as seen in the figure of species a {Ophio- 

 pluteus Henseni). The transverse rods are very short, pipe-shaped, thick- 

 ened; the end rods are even shorter, also somewhat widened. All the rods 



1) Th. Mortensen. Studies in the development of Crinoids. Papers from the Depart- 

 ment of Marine Biology. Carnegie Inst. Washington. Vol. XVI. 1920. 



Th. Mortensen. Notes on the development and the larval forms of some Scandinavian 

 Echinoderms. Vid. Medd. Dansk Naturh. Foren. 71. 1920. p. 151. 



Also the eggs of Ophioderma brevispina Say float, but in this case it is the large content 

 of yolk which makes them float, no special structure of the egg membrane apparently serving 

 as a floating apparatus. (Casw. Grave. Ophiura brevispina. Mem. Biol. Lab. Johns Hop- 

 kins Univ. IV. 5. 1900. — Ophiura brevispina. II. Journ. of Morphology. 27. 1916). 



