173 



accumulations of nuclei are seen which, no doubt, represent the posterior 

 coelomic vesicles. There is an indication that one of them is in connection 

 with the ectoderm, but it cannot be stated definitely to be the case. — 

 It is very unfortunate that the material available does not allow a detailed 

 study of the process of formation of the hydrocoel and the coelomic vesicles, 

 so that a comparison with Grave's researches on 0. brevispina might be 

 made. Especially his statement^) that the epigastric coelom originates as 

 a separate invagination from the ectoderm is so remarkable that it would 

 have been of the greatest interest to see, whether it originates in the same 

 peculiar way in this species also. There is, as stated above, an indication 

 that it really does so, but it can by no means be ascertained as a fact. 



The specimen represented in PI. XXXI, Fig. 6, although of the same 

 age, is somewhat more advanced in its development, the hydrocoel having 

 formed the five primary lobes. The small sac lying at the upper end of 

 the stomach apparently is the right anterior coelomic vesicle; the posterior 

 coelomic vesicles could not be discerned. — 



At the age of 40 — 45 hours the larva has reached its full development 

 and has a very characteristic shape (PI. XXXI, Figs. 1 — 2). The anterior 

 end forms an elongate, rounded lobe with a ciliated band around it, 

 situated a little below its middle; below and above the band the lobe is 

 narrowing, at the point there is a large ciliated tuft, which is of a bright 

 pink colour. In the middle of the posterior part of the body there is another 

 spot of pink colour, larger than the apical spot, but less intense. It coincides 

 with the central part of the young Ophiurid, which has already developed 

 its first pair of tubefeet. 



Two ciliated bands are found in the posterior, main part of the body, 

 one at the level of the mouth of the developing Ophiurid, the other at the 

 posterior end. Both are incomplete; the band in the middle of the body 

 consists of a median part on the dorsal side and two lateral parts, while 

 the posterior band consists of two parts, a larger one on the right side 

 and a smaller one to the left. Possibly the posterior band has been complete, 

 surrounding a posterior lobe in a former stage, before the developing 

 Ophiurid has reached so far in its development. That the band in the 

 middle of the body is not the broken up rests of a formerly complete band 

 would seem evident from the fact that in the next stage figured (PI. XXXI, 

 Fig. 3), of a larva 2V2 days old, the band is much more developed and 

 nearly complete across the dorsal side. 



At the age of 40 — 45 hours the skeletal plates begin to appear. In the 



1) Caswell Grave. Ophiura brevispina. II. An Embryological Contribution and a Study 

 of the effect of the Yolk substance upon development and developmental processes. Journ. 

 of Morphology. Vol. 27. 1916. 



