422 EETIEWS. 



So much, then, for the old classes, Acalephse and Polypi, groups 

 by no means susceptible of accurate definition. The Acalephse of 

 Cuvier and Eschscholtz differ, iudeed, much more among themselves 

 than do many of them from the Polypi, presenting little mutual 

 agreement save in their Coelenterate orgauization, gelatinous texture, 

 and oceanic habit. Yet more difficult is it to point out such ana- 

 tomical features as are common to the various forms of Polypi, ex- 

 cepting, of course, those which justify their being placed in the same 

 sub-kiugdom. It is not correct to say that, like the Acalephse, they 

 observe a certain similarity in their mode of life, considerable differ- 

 ences in this respect prevailing even among closely allied genera. 

 But, were it otherwise, so purely physiological a tendency would avail 

 the systematist little. For since the same essential may coexist with 

 very different adaptive characters, community of habit, apart from 

 resemblance in plan of structure, affording no safe ground for classi- 

 fication, it is plain that the two groups of Coelenterata to which re- 

 ference has been made can no longer be regarded as natural. What 

 others, then, have been substituted in their stead ? 



To Professor Eapp, of Tubingen, must be ascribed the merit of 

 having first pointed out the way towards a right solution of the diffi- 

 culty now under consideration. In a small quarto treatise, 29 pub- 

 lished at Weimar in 1829, this zoologist laid down with great clear- 

 ness and precision, the limits of two primary sections of the class 

 Polypi, bearing the names of Exoarii and Endoarii. The Exoarii 

 contained those Polypes which developed their generative elements 

 within external processes of the body wall ; the Endoarii those whose 

 reproductive organs were lodged in the interior of the general cavity. 

 This last division corresponded to the second and third orders in the 

 arrangements of Johnston and Milne Edwards. 



At a much later period both Dana 30 and Milne Edwards, 31 ac- 

 knowledged two groups of Polypi equivalent in systematic value to 

 those of Eapp. Previously both Owen and Ehrenberg had raised 

 these divisions to the rank of classes ; the Exoarii being designated 

 Hydrozoa, 38 while the term Anthozoa, first applied by Ehrenberg to 

 all the Coelenterate Polypes, was restricted to the Endoarii. 



29 Ueber die Polypen im Allgemeinen unci die Actinien insbesondere. 



30 Eeport on Zoophytes, U. S. Exploring Expedition, 1846. 



31 In the Introduction to their Monograph of the British Fossil Corals, published 

 by the Paleeontographical Society, 1850, Milne Edwards and Haime divided the 

 Polypi into two equal sub-classes, Corallaria and Hydraria. The Corallaria corres- 

 pond to the orders Zoantharia and Alcyonaria in the earlier arrangement of Milne 

 Edwards, the Hydraria to his Sertulairiens. 



32 Lectures on the Invertebrate Animals, 1843, p. 86. 



33 In addition to the Hydrozoa of Owen, Rapp included among his Exoarii a 

 small group of Polypes, to which he gave the name of Milleporen. It is not easy 

 to determine the precise sense in which the latter is used, but, obviously, Ilapp 

 meant not thereby to designate the forms now placed in the family of Milleporida). 



