237 



cavit3^ Further it is a fact that I he growth of [Iw hydrocoel in Ophiurid- 

 larv<e lakes phice in differenl ways. In the hirva of AmphiiiKi liUjuimis 

 {Opbiophileus manrus) e. g. it ijrows iqiwards over the nioulli, the closing 

 of llie hydrocoel ring Inking place at the lower end of the pharynx; in 

 the huvii of Ophiura <tlbid(i {Ophiopluleus paradoxus) the growth proceeds 

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

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

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

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

 as regards the formation of the hvdrocoel ring. These facts at anv rate 

 show this much, that we nuisl be vtM\ 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 MacHride's theory 

 of the origin of the amnion of I^chino])lutei 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 (he hnva of Pcronclla 

 Li'suciiii nor apparently in llcliocidaris en/lhroiiramivd until at a rathei- 

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

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

 it being probably a nuxlilication caused by the exceptioiud conditions ob- 

 taining in these larvaj. 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 nu)rphoIogy. 



On discussing the (piestion of the original type of the h^chino- 

 derm-larva; Caswell (irave^) comes to the conclusion that the larva? 

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

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

 far out of the ])ath of phylogeny, as a result of their independent life. 

 To this type of development the specialized larva^ tend to return at the 

 time when their fiee-swimming life is given up." He nutkes an attempt 

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

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

 texlfiguri' 11. 1).). The atlem|)t does not ajjpeal 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 larva' of this type occur in various groups of the 1-Ahinoderms. 

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



') Caswell Gravi'. On the occurreiu-i- atiiong Eiliiiioilonns of larva' willi cilia arraiigcil 

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



