162 ESSAY ON CLASSIFICATION. 



hardly expect a very close coincidence in this respect 

 between animals whose structure is so simple. The 

 prevalence of Rugosa and Tabulata in the oldest deposits 1 

 appears in a new light, since it has been known that the 

 Tabulata are Hydroids, and not genuine Polyps. 2 



The gradation among the orders of Echinoderins is 

 perfectly plain. Lowest stand the Crinoids, next the 

 Asterioids, next the Echinoids, and highest the Holo- 

 thurioids. Ever since this class has been circumscribed 

 within its natural limits, this succession has been con- 

 sidered as expressing their natural relative standing, and 

 modern investigations respecting their anatomy and 

 embryology, however extensive, have not led to any 

 important change in their classification, as far as the 

 estimation of their rank is concerned. This is also 

 precisely the order in which the representatives of this 

 class were successively introduced upon the earth in 

 past geological ages. Among the oldest formations we 

 find pedunculated Crinoids 3 only, and this order remains 

 prominent for a long series of successive periods ; next 

 come free Crinoids and Asterioids ; next Echinoids, 4 the 

 successive appearance of which, since the Triassic period to 

 the present day, coincides also with the gradation of their 

 subdivisions, as determined by their structure ; and it 

 was ngt until the present period, that the highest Echino- 

 derms, the Holothurioids, assumed a prominent position 

 in their class. 



Among Acephala there is no more uncertainty re- 



1 See MILNE-EDWARDS and HAIME, System der Asteriden ; Braunschweig, 

 q. a., p. 44. 1842, 4to. fig. MULLER, (J.,) Ueber 



2 Comp. the notes, pp. 34 and 107. den Bau der Echinodermen ; Berlin, 



3 MILLER, Crinoids, q. a. D'On- 1854, 4to. TIEDEMAN, (FR.,) Anato- 

 BIGNY, q. a. J. HALL, q. a. Aus- mie der Rohren-Holothurie, des See- 

 TIN, q. a., p. 145. igels, etc.; Landshut, 1817, fol. fig. 



4 See the works q. a., p. 145; also, VALENTIN, (G.,) Anat. du genre Echi- 

 MULLEE, (J.,) and TROSCHEL, (F. H.,) nus ; Neuchatel, 1842, 4to. 



