79 



excepting only the antero-lateral arms, which are well developed, though 

 small, in the four first species. The postoral rod is fenestrated, the rudi- 

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

 typical basket-structure. The posterior transverse rod has a very pecuUar 

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

 young sea-urchin) is present in species e— f ; whether it is present also in 

 the other species remains uncertain. The vibratile band is not strongly 

 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 

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

 uish with certainty the rectum and the anal opening. 



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

 lateral arms of Ophiurid larvae, to such a degree even that in young stages, 

 where the characteristic skeletal parts of Echinoplutei (dorsal arch, post- 

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

 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 postoral 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 the strong development of the musculature 

 in the species 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. Fig. 2). The single, well preserved, specimen of 

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

 by Mr. Blegvad in the West Indian Sea, at 18°59' N. 65°05' W. (a little 

 to the North 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 body (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 httle 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 band forms a simple 



