86 CALAMOCRINUS DIOMED.E. 



we have neither pala3ontological nor embryological evidence that the Cjs- 

 tideans i-epresented the primordial type of Echinoderms, does not appear 

 to me to be sustained by the facts, any more than the statement that 

 on embryological grounds we have far more reason to look upon the 

 Holothurians as the representatives to-day of the most primitive type of 

 Echinoderms.* 



The assertion that the Holothurians were the stock from which the 

 other Echinoderms were derived must always remain a sheer assumption. 

 There is nothing thus far known of the embryology of that group which 

 lends any support to that view ; on the contrary, the primordial type of 

 Echinoderms, as we may imagine it from their embryology, is a sac-like 

 animal, with plates arranged irregularly, much as is the case in the Cys- 

 tideans, with a simple ambulacra! system arranged around a central open- 

 ing. Such a semicystid embryo has been observed in Ophiurans, Starfishes, 

 Echini, and Crinoids, and, with the addition of a larger number of limestone 

 plates, is also represented in the Holothurians. See the young stages of 

 Psolinus as figured by Kowalevsky and Metschnikoff, and of Cuvieria, of 

 which I have figured the original Y-shaped rods.f 



The Sarasins ^ look upon the pentagonal arrangement of the plates and 

 the ambulacra as a secondary structure due to the longitudinal nerves 

 and muscles, the formation of rings of five or ten plates being due to the 

 uniting of smaller irregularly arranged plates, analogous to the coalescence 

 of primary ambulacral plates into large plates. This statement I do not 

 understand, as original centres of calcification rarely anchylose except in 

 older stages of growth. 



The five perisomic oral plates in some species of Psolus are very similar 

 to those of Hyocrinus ; but the fact that they are replaced in some species 

 by many plates instead of five by no means proves that they have coalesced. 

 We look upon five as the original number of plates, which are devel- 

 oped in succession, and one of which may take a very great development 

 and retain its prominence. 



The Sarasins follow very much the same course of reasoning as Neu- 

 mayer and Semon in speaking of the suranal plate of Salenia as a Crinoid 



* See Semon, R., Die Entwickelung der Synapta digitata u. die Staramesgeschichte der Eehinodei- 

 men. Jenaisciie Zeitsclir. f. Naturwiss., XXII., 1888. 

 t Embryology of the Ecliinoderms, 1864, Fig. 38. 

 t Paul und Fritz Sarasin, Ergebnisse naturwissenschaftlicher Forschungen auf Ceylou, Drittes 



Heft. 1888. 



