178 



the time, at present, for this apparently very difficult study. It must suf- 

 fice at the present occasion to state only that there is formed at the place 

 destined to form the oral side of the Ophiurid an accumulation of nuclei, 

 like a germinal disk, from which the formation of all the main structures 

 of the Ophiurid proceeds, while the large yolk mass remains undifferent- 

 iated, simply as a reservoir of food, covered only by a thin ectodermal 

 coat. The arms of the young Ophiurid gradually as they grow are folded 

 up over the yolk mass (PI. XXXI, Fig. 9). 



The embryos proved unfavourable objects for a detailed study of the 

 development of the oral skeleton. The usual primary plates are formed 

 in the skin of the dorsal side. There is no trace of a larval skeleton. - 

 The embryos are not liberated until they have reached a comparatively 

 large size, with ca. 10 armjoints; the paired infradental mouth papillae 

 have then already appeared; they do not appear till the teeth have been 

 fully formed, the latter being among the first parts of the oral skeleton 

 to appear. 



That this type of development will prove to be of more general occur- 

 rence in viviparous Ophiurids can hardly be doubted, viz. in such forms 

 as have large eggs, rich in yolk; but not in all of them. In Stegophiura 

 nodosa (Ltk.), the eggs of which are 0.5 0.6 mm in diameter and rich in 

 yolk the cleavage is total and regular. A detailed study of the develop- 

 ment of these two Ophiurids, both having large, yolk-laden eggs, but one 

 of them having superficial, the other total and regular cleavage, would, 

 evidently, be of exceptional interest. This is, however, entirely out of the 

 scope of the present work. But it is worth emphasizing that Amphipholis 

 squamata is so far from representing the usual type of development among 

 viviparous Ophiurids that it appears in reality to be exceptional in this 

 regard. 



The interesting observation that Amphiwa vivipara is hermaphroditic 

 like Amphipholis squamata may be recalled here. It led to an examination 

 of other viviparous Ophiurids which resulted in the establishment of the 

 surprising fact that out of the twenty viviparous forms of Ophi- 

 urids known sixteen are hermaphroditic, three alone having 

 separate sexes (one remaining unknown as regards its sexual characters). 

 This fact, the more conspicuous since not a single case of hermaphroditism 

 is known otherwise in Ophiurids, shows that there is some relation between 

 viviparity and hermaphroditism in Ophiurids, though it remains enigma- 

 tical, wherein this relation consists. (Comp. the author's paper "On herma- 

 phroditism in viviparous Ophiurids"). 



