26 ON THE EMBRYOLOGY OF ECHINODERMS. 



istic of the embryos of Echinoderms are carried to a great prominence in the different 

 families. These are, to give a few examples, the great development of the marginal 

 plates of the arms, the character of the S23iues on the abactinal area, and the presence of 

 pointed tentacles in the adult Starfishes. Ophiurans, on the contrary, are a peculiarly 

 embryonic order. They never develop interambulacral plates, which, as we shall see in 

 Asteracanthion,* develop quite late in the life of the young Starfish. What is very 

 remarkable in all the young Echinoderms is their Crinoidal character. A stem added 

 to a very young Starfish, Ophiuran, or Echinoid recalls to us many of the forms with 

 which we are familiar in the palseontological history of our earth, and I have no doubt 

 that a comparative study of the innumerable Crinoids known, and of the living and 

 fossil Echinoids, Starfishes, and Ophiurans, will bring out many more points of interest 

 than have been here alluded to, and give us a correct idea, not only of the nature of 

 the orders, but also of the families which compose them. I have here pointed out a 

 few of the characters which distinguish the different orders of Echinoderms ; I shall 

 endeavor to adopt the same method, to show how far what we know of the embryology 

 of Echinoderms will assist us in forming a true conception of the classification of 

 Radiates, reserving closer comparisons between the development of Acalephs and Echi- 

 noderms for another occasion, when I shall treat more fully than I have room for here 

 of the development of the Ctenophora;, which gives us the connecting link between 

 the Polypoidal and Echinodermal mode of development. The division of Coclenterata, 

 proposed by Leuckart in contradistinction to Echinoderms, does not correspond to any 

 natural distinction we can draw between the mode of development of Echinoderms on 

 the one side, and that of Polyps and Acalephs on the other. The mode of dcAelop- 

 ment of Polyps differs more from that of the higher Acalephs — the Ctenophorae, for 

 instance — than that of the Ctenophora? differs from the Echinoderms. But what is a 

 fatal objection to the division proposed by Leuckart is the appearance at the earliest 

 stages of development of definite numbers of spheromeres, whether we deal with a 

 Polyp, an Acaleph, or an Echinoderm ; and these spheromeres are not simply analo- 

 gous parts, as the tentacles of the embryos of some Annelids and the arms of the 

 plutean state of Echinoderms, but are strictly homologous, showing plainly that 

 the same plan underlies the mode of development of these three classes, though it is 

 carried out in such different ways in Polyps, Acalephs, and Echinoderms, and that 

 the separation of the great type of Radiates into two branches, as proposed by 

 Leuckart, is an artificial division which has no true foundation in nature. 



• See Vol. V. of Contrib. Nat. Hist. U. S., by L. Agassiz. 



