50 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



The smallest larvae are 0.9 mm., the largest 1.8 mm. in length. The egg- 

 membrane evidently has been ruptured a long time before the larvae reach 

 that size. In embryos sectioned in situ within the marsupia, parts of what 

 would appear to be an outer membrane are seen close to the skin; otherwise 

 no trace of an egg-membrane can be observed. This accords well with the 

 fact that the larvae grow to such large size that they must necessarily be 

 assumed to obtain nourishment from the mother animal. That could hardly 

 pass through an egg-membrane. As there is no mouth, it could take place 

 only by absorption through the skin. 



It is worth mentioning that on embedding the larvae in paraffine the skin 

 would always break in various places, in spite of the most careful treatment ; 

 but when they were embedded lying undisturbed within the marsupia their 

 skin never broke. 



The shape of the larva (plate xxiv, figures 1 to 3) is generally more or 

 less irregular, on account of the pressure in the marsupium, but upon the 

 whole it is somewhat flattened and slightly concave on the ventral side, 

 while the dorsal side is more arched, the hind end generally more so than the 

 anterior end. On the ventral side there is in the anterior end a more or less 

 distinct arcuate depression, the convexity being directed towards the ante- 

 rior end. This represents the suctorial disk. There is no apical pit. Thevestib- 

 ulary invagination is narrow. In the apparently most normal-shaped larvae 

 (plate xxiv, figure 1) there is a shoulder-like prominence on each side at the 

 anterior end; but whether this is a typical feature can not be ascertained. 



One larva (plate xxiv, figure 3) exhibits a peculiar feature in having a 

 slender prominence, like a thin stalk, on one side, suggesting that it had 

 coalesced with the wall of the marsupium; but as nothing similar was 

 observed in any other embryo it can not be a normal feature. 



A very interesting fact is the total absence of ciliated bands in this larva. 

 Neither does a general ciliation appear to exist; only in the suctorial disk 

 and the vestibulary invagination are cilia distinctly seen. The ectoderm is 

 much thickened in the whole of the anterior end, gradually thinning out 

 towards the posterior end (compare the longitudinal sections, plate xxiv, 

 figures 4 to 9) ; the extension of the thickened portion differs to some degree, 

 as seen in the figures quoted; in the specimen represented in figures 4 to 6 

 it passes below the hydroccel, in the other specimen (figures 7 to 9) it is con- 

 fined to the part beyond the stomach and the hydroccel. The histological 

 details are hard to discern in the thick part of the ectoderm; it is a mass of 

 nuclei, intermingled with glandular cells. In the part where the transition 

 to the thin part takes place, an outer layer of small, closely aggregated nuclei 

 and an inner layer of larger nuclei can be discerned, between which is a mass 

 of irregular thread-like structures (plate xxvi, figure 9). It can hardly be 

 doubted that this corresponds to the glandular cells found in the skin of 

 other Crinoid larva? and it would appear that the inner, larger nuclei belong 

 to the glandular cells. Full certainty can not be acquired on account of the 



