179 

 Ophionotus hexactis (E. A. Smith). 



PI. XXXII. 



In the paper quoted above "On hermaphroditism in viviparous Ophi- 

 urids" (p. 17) the observation was mentioned that this species, in spite 

 of being viviparous, has a true, well formed larva, the develop- 

 ment being intraovarial. While in that paper the hermaphroditic cha- 

 racter of the species was established, I may here take the opportunity of 

 giving a description of the remarkable intraovarial development of the no 

 less remarkable larva. 



The previous observers, Studer^) and Lyman^) assumed that the young 

 were enclosed in the much widened and very thin-walled bursse, or "rather, 

 perhaps, in pockets leading out of the bursse" ; this latter expression shows 

 that Lyman has seen something of the real fact; the understanding of 

 the whole astonishing fact could, however, not be gained through simply 

 examining how the large young are lying within the parent specimen. 

 Only an examination of the ovaries could disclose the fact, almost unique 

 among the Echinoderms^), that the young are really lying within the 

 distended ovaries. 



The structure of the young ovaries differs somewhat from that generally 

 observed in the female gonads of Ophiurans. While generally they are 

 completely filled with eggs so as to be solid organs, they are in 0. hexactis 

 hollow vesicles, the eggs attached to the walls not nearly filling out their 

 lumen (PI. XXXII, Fig. 9). The eggs are of various sizes, one or a few of 

 them being larger than the rest ; the size of the ripe egg is about 0.2 mm. 

 When ripe the egg falls into the cavity of the ovary, but it does not pass 

 out into the bursa, as should be expected; it remains within the ovary 

 and, after being fertilized there, goes on developing, the embryo passing 

 its whole development within the ovary. 



Gradually as the embryo grows, the walls of the ovary are distended. 

 The other eggs remain attached to the wall as flat buttons, gradually wider 

 and wider apart from one another. In the same time they gradually di- 

 minish in size and ultimately they disappear completely. It seems beyond 

 doubt that they must serve as nourishment for the young contained within 

 the ovary. Traces of eggs on the ovarian walls may still be seen when the 

 young has about 4—5 armjoints (PI. XXXII, Fig. 8). As the young grow 



1) Th. Studer. tJber Geschlechtsdimorphismus bei Echinodermen. Zool. Anzeiger. 1880. 



^) Th. Lyman. Ophiuroidea of the "Challenger". 1882. p. 41. PI. XLV, Fig. 1; PI. 

 XLVII, Fig. 2. 



») Intraovarial development is known to occur only in one other Echinoderm, Chiro- 

 dota contorta Ludw. Ludwig. Holothurien d. Hamburger Magelh. Sammelreise. 1898. 



p. 77—81. 



23* 



