ON THE EMBRYOLOGY OF ECHINODERMS. 19 



and July for weeks without noticing any striking changes. These Ophiuran Lirvaj 

 resemble very closely the Pluteus of Ophiothrix fragilis and the Pluteus himacidatus 

 of Trieste, figured by Miiller in his Memoir on the Ophiuran Larvse of the Adriatic* 

 Fig. 31 resembles more Miiller's fig. 1, Plate VII. of the Memoir just quoted, than any 

 other. The order of appearance of the arms of this Pluteus agrees with his observa- 

 tions on the younger stages of the Pluteus of Ophiothrix fragilis. (Memoir V., Plate 

 VI. figs. 8-12.) This has been traced in a specimen considerably younger than the 

 one figured here, resembling Miiller's figure of Pluteus bimaculatus (Plate I. fig. 1, !Me- 

 moir v.), with the exception of the different degree of development of the arms C and 

 D of that figure. In this younger Pluteus, the arms corresponding to e' of fig. 31 were 

 the longest ; next the arms e'^, then e"', the arms e" scarcely projecting from the curve 

 joining e" and e'. The arms e' in the adult larvae are twice as long as the others 

 (fig. 31), while the other arms, e", e'", e", are all nearly equally developed. With the 

 exception of this difference in the proportion of the arms, the younger larvae did not 

 differ in the essential parts from the one which is figured here. The most complete 

 histories which Miiller has given us of the development of any Echinoderms are those 

 of Pluteus bimaculatus and of Ophiothrix fragilis. He gives us not only the complete 

 history of the changes of the larva, but follows the young Ophiuran after it has 

 absorbed its calcareous framework. Here, again, the figures of Miiller seem to con- 

 tradict his text ; he says the framework is dropped, but his figures show that, on the 

 contrary, nothing is lost, that every part of the Pluteus is absorbed by the Echino- 

 derm. See Miiller's figures of Memoir V., Plates IV., V., VI., VII., VIII. Fig. 5, 

 Plate VII. agrees exactly with the stage represented by me in figs. 32, 33 ; the long 

 arms e' are cut off for want of room ; every other arm can be traced, Avitli its ex- 

 tremity perfect, disappearing gradually into the substance of the Ophiuran. This 

 Pluteus is quite transparent, and did it occur in sufficient numbers it would be as 

 favorable a species to follow the development of the Echinoderm as the Brachiolaria 

 of our Starfishes. 



The greater similarity of the Ophiuran larva to Echinoids than to Starfishes is 

 something very peculiar. The same thing we find again repeated for two of the other 

 orders ; the larvae of Holothurians and of Starfishes resemble each other to a remark- 

 able extent, in the character of the arms, the absence of rods, and the aspect of the 

 water-tubes. In Ophiurans and Echinoids this resemblance is still closer. The arms 

 are supported by rods in both, the water-tubes are small, and tlic general outline of 



" MuLLER, J. Ueber die Ophiurenlarven des Adriatischen Meeres. Fiinfte Abhandlung. Berlin, 1852. 



