PLAX OF DEVELOPMENT OF ECHINODEKMS. 77 



and Starfish larvse are developed from the vihratile chord developed round 

 the anus. Nothing is more characteristic of the Echinoderms among Ra- 

 diates than the isolation of the digestive cavity by means of distinct walls. 

 This feature is so strongly marked that a larva can he recognized as an 

 Echinoderm larva before its radiate characters are developed. It is only 

 later that the circular tube, the water-system, is formed, while the ciliary 

 appendages, which have nothing to do with the formation of the Echino- 

 derm, make their appearance later still long after the first rudiments of 

 the Echinoderm (the water-tubes) are present. 



It seems to me that the different modes of development in Holothu- 

 rians. Echini, true Starfishes, Ophiurans, and Crinoids, different as they are 

 apparently, may easily be reduced to a single type. [See the Memoirs 

 of Metschnikoff on the affinities of Echinoderms, in Siebold's Zeitschrift for 

 1874. Since this paper was written Haeckel has put forth his views of 

 the relationship of the Sponges and Coelenterates, and of the Echino- 

 derms and Worms. As the whole subject is intimately connected with the 

 history of Tornaria and Balanoglossus. I would refer to my Memoir* for 

 an analysis of the modifications which these views are likely to bring 

 about regarding the classification of Echinoderms and of Coelenterates ; 

 also to my Memoir on the Embryology of Ctenophorae, Mem. Am. Acad. 

 1874.] We have in Ophiurans two different modes of development, — one 

 by means of the Pluteus, the other by means of the viviparous mode t)f de- 

 velopment observed by Krohn and Schultze. We have two similar modes of 

 development in the Starfishes, — the one as observed by Sars and Agassiz 

 in Echinaster, the other in which the embryo assumes the shape of a Bi- 

 pinnaria or Brachiolaria ; and, finall}^, in the Holothurians we have these 

 two modes represented by the Auricularia type and the type of the 

 "Wurmformige Holothurienlarve." [See also for Echini Thomson's paper 

 in Journal Lin. Soc. and A. Agassiz, Viviparous Echini from Kerguelen, 

 Proc. Am. Acad. 1876.] The difference between these two modes seems 

 to be one of time ; in one case, the eggs are retained by the parent 

 until they have passed through many of their changes, and are freed 

 in a stage corresponding to that of our young Echinoderm after it 

 has resorbed its Pluteus, its Brachiplaria, or its Auricularia. L^ the 

 other case the e«i:o; <roes throuo^h all these changes after it has left the 

 parent, developing this complicated system of arms, which seems to be 



* A. Agassiz Balanojilossus and Tornaria in Memoirs American Academv, 1873. 



