79 



excepliiiii; only llu- aiih-io-laU'ial arms, which are well develo])e(l, lhouf>li 

 small, ill the lour lirsl species. The posloral rod is fenestrated, the rudi- 

 mentary posterodorsal rod is simple. There is a recurrent rod, but no 

 typical baskel-striKiiiic. Tiu' posterior transverse rod has a very peculiar 

 bow-shape. A small, unpaired, posterior process (perhaps a spine of the 

 youni* sea-urchin) is |)resent in species e^-f ; whether it is |)resent also in 

 the other species remains uncertain. The vibratile band is not stronf»ly 

 developed on the body, in accordance with the reduced state of the arms: 

 there are no vibratile lobes or epaulets. In species a-^d the preoral lobe 

 has the typical shape; in species e— f it bends forwards so as to cover the 

 mouth and the oral area. In the two latter species the esophagus appears 

 lo be folded. The stomach is very wide; it has been impossible lo disting- 

 uisii with certainty the rectum and the anal openini*. 



The posloral arms are directed outwards so as to recall the postero- 

 lateral arms of ()])hiurid iarv;e. lo such a degree even tiuil in young stages, 

 where I he characteristic skeletal parts of Echinoplulei (dorsal arch, ])Ost- 

 erior transverse rod) have not yet been formed, it is really very hard lo 

 see, whether it is an Echinoid- or an Ophiurid-larva. From the ventral 

 transverse rods, which are more or less transformed, a supplementary 

 transverse rod proceeds, serving for the attachment of an adductor muscle, 

 which (in the species e— f) together with a similar muscle on the dorsal side, 

 serves as antagonist to a powerful abductor muscle connecting the ends of 

 the short, widened body rods. By means of this muscular apparatus the 

 long posloral arms can be moved actively and thus apparently serve as an 

 active locomotor organ, besides the usual ciliary movement of the vibratile 

 bands. The immense length of the postoral arms together with the active 

 swimming movements, which I he strong development of Ihe musculature 

 in I he s|)ecies e and f indicate almost certainly to take place, make this 

 larva one of the organisms most highly adapted to pelagic life. 



Species a. (PI. XIII. l"'ig. 2). The single, well preserved, specimen of 

 this species , which was found in a plankton sample from the surface taken 

 by Mr. Blegvad in Ihe West Indian Sea, at 18°59' X. 65°05' W. (a Httle 

 lo Ihe Xorth of the Virgin Islands), (l/III. 1911), is a young one, not yet 

 having the hydrocoel distinctly differentiated. The postoral arms are ca. 

 1.5 mm long, or nearly 7 times the length of the boily (the point is broken, 

 so that the total length remains unknown, but it is not likely to be much 

 more than the preserved part). The antero-lateral arms are well developed, 

 a little more than body length. They are somewhat widened at the base 

 and with a slight widening at the point. The preoral ciliated band is well 

 developed and forms a beautiful curve: the postoral ])and forms a simple 



