OPHIURA BREVISPINA 437 



representation of the correlation phenonuMioii imdcM- considera- 

 tion for several reasons, the most important of which are (Numer- 

 ated below, but it is the best that can l)e constructed from the 

 data now available. The niunber of species for which both the 

 size and character of mature egg and length of larval life are 

 known is few. Certain species, of which these facts are known — 

 Asterina (McBride), Cribrella (Masterman) and Solaster (Gem- 

 mill) — can not be placed in tlie scheme because in the first two 

 the eggs and larvae are brooded and are hence freed from the 

 larval struggle for existence, and, in the last, the adult form 

 possesses nine instead of five arms and must therefore require 

 much more energy and material for its development than is 

 required in five rayed forms. 



All species of star-fish are, for another reason, excluded froju 

 the correlation curve. With respect to volume of living or 

 ground substance, the eggs of Astei'ids seem to belong to a seiies 

 very different from that which includes the eggs of Echinoids 

 and Ophiurids, the former containing a consistently greater 

 volume of living substance. This fundamental difference be- 

 between the eggs of the two series is evident from a comparison 

 of the eggs of Asterias forlx^sii and Toxopneust(>s vai'iagatus. 

 Both of these eggs are semi-transparent and contain the mininmm 

 quantity of yolk required for the formation and establishment 

 of an independent larval form. In all physical pro]ierties, 

 except that of the size, they seem to be similai", tlu^ former 

 having a diameter of about 0.15 mm. the latter about COOf) 

 mm. (text fig. 3, F and E.) In each case the ratio between 

 volume of ground substance and quantity of stored yolk is 

 probably the same. Both produce highly specialized larvae 

 with long periods of developjnent. 



The eggs of Asterids may therefore be considered to consti- 

 tute a series distinct from, but parallel to, that of Echinoids 

 and Ophiurids in which an ogg like that of Asterias stands at 

 one end and an egg like that of Cribrella at or near the other. 



Temperature is a factor which also considerably affects the 

 rate of development of echinoderm eggs, and, since echinoderms 

 develop under widely diffor(Mit and fluctuating temperatures, 



