237 



cavity. Further it is a fact that the growth of the hydrocoel in Ophiurid- 

 larvae takes place in different ways. In the larva of Amphiura filiformis 

 (Ophiopluteus mancus) e. g. it grows upwards over the mouth, the closing 

 of the hydrocoel ring taking place at the lower end of the pharynx; in 

 the larva of Ophiura albida {Ophiopluteus paradoxus) the growth proceeds 

 in the opposite direction, the closure of the hydrocoel ring taking place 

 above the mouth. (Comp. PI. Ill, Fig. 4; PI. IV, Fig. 28 and PI. VI, Fig. 40 

 of Chadwick's Memoir on the Echinoderm-larvae). The larva of Ophio- 

 thrix fragilis appears to be about intermediate between these two forms, 

 as regards the formation of the hydrocoel ring. These facts at any rate 

 show this much, that we must be very cautious in drawing such important 

 conclusions from facts derived from the study of only one single form. — 

 With this I do not mean to deny the possibility that MacBride's theory 

 of the origin of the amnion of Echinoplutei may be right. On the contrary, 

 it appeals to me as a very reasonable suggestion. Also a comparison with 

 the vestibulum of Crinoid-larvse would seem to lend support to this theory. 

 The fact that the hydropore is not formed in the larva of Peronella 

 Lesueuri nor apparently in Heliocidaris erythrogramma until at a rather 

 advanced stage of metamorphosis, may be recalled here. I would, how- 

 ever, not regard this fact as being of greater morphological importance, 

 it being probably a modification caused by the exceptional conditions ob- 

 taining in these larvae. In the same way I would not regard the unusual 

 way in which the enterocoel develops in these forms as of essential im- 

 portance from the point of view of comparative morphology. 



On discussing the question of the original type of the Echino- 

 derm-larvge Caswell Grave^) comes to the conclusion that the larvae 

 with transverse ciliated rings (Antedon, Cucumaria) represent the primitive 

 condition from which the other larvae have been specialized "and carried 

 far out of the path of phylogeny, as a result of their independent life. 

 To this type of development the specialized larvae tend to return at the 

 time when their free-swimming life is given up." He makes an attempt 

 to show, how the ciliated rings were useful to the free-swimming animal 

 not only as organs of locomotion, but also as organs of feeding (comp. his 

 textfigure 11. b.). The attempt does not appeal to me as very successful, 

 apart from the fact that it could hardly always have been, as he states, the 

 two anterior rings that were lost when fixation on the bottom took place. It 

 is true that larvae of this type occur in various groups of the Echinoderms, 

 being even apparently the rule in ComatuUds and Dendrochirotes. The fact 



1) Caswell Grave. On the occurrence among Echinoderms of larvse with cilia arranged 

 in transverse rings, with a suggestion as to their significance. Biol. Bull. V. 1903. p. 183. 



