237 



cavity. Further il is a fact that the growth of tin- hydrocoel in Ophinrid- 

 larv;e takes place in dill'erenl ways. In the larva ol' Ampliiuni /ilifonnis 

 (()pliit>f)lul<'iis indin-nx) e. g. it grows upwards over I he mouth, I he closing 

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

 the larva of Ophiura dlbidd (()i)hiof)luleus pdnnlo.rus) the growth proceeds 

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

 above the mouth. (Comp. PI. III. Fig. 4; PI. IV, Fig. 28 and PI. VI, Fig. K) 

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

 Ihri.i fnHjilis 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 cautions 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 Kchinoplutei may be right. On the contrary, 

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

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

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

 Lcsiu'iiri nor apparently in Hcliocidaris 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 larva 1 . 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 Kchino- 

 derm-larv;e Caswell (irave 1 ) comes to the conclusion that the larva- 

 with transverse ciliated rings (Anlcdon, C.ucumaria) represent the primitive 

 condition from which the other larva? 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 specialixed larva- tend to return at the 

 lime when Iheir free-swimming life is given up." He makes an allempt 

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

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

 lexlfigure 11. I).). The allempl does not appeal lo me as very successful. 

 aparl from the lad thai il could hardly always have been, as he states, the 

 I wo anterior rings thai were lost when fixation on the bottom look place. II 

 is true that larva 1 of Ihis type occur in various groups of the Kchinoderms. 

 being even apparently the rule in C.omatulids and Dendrochiroles. The fad 



') Caswell Grave. On tin- occurri-nri- aiming Kcliinmlrnns nt larva' with cilia 

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



