Chap. II. SUCCESSION OF ACALEPHS. 127 



of their lowest order; and we have just seen that the Tabuhata occupy an inferior 

 j)osition in that lowest order. 



Assuming that Rugosa are Hydroids also, the question of their standing in their 

 order is not difficult to determine, if we take into consideration the general char- 

 acter of the class, and its relations both to the class of Polyps and to that of 

 Echiuoderms. I have already alluded to the analogy between the Hydroids and 

 the Polyps, and to that between the CtenophorsB and Echiuoderms. Starting from 

 this fact, let us see what are the elements of superiority and inferiority among 

 the Radiates, at the two ends of the type. In Polyps we distinguish two orders, 

 the Actinoids and the Halcyonoids. Taking their whole structure into consideration, 

 the Actinoids with their simple tentacles and the indefinite repetition of similar 

 parts in most of them, are, unquestionably, inferior to the Halcyonoids with their 

 eight-lobed tentacles and invariable eight spheromeres. Now, among Halcyonoids 

 there are no simple individuals : all the types of this order consist of compound 

 communities, while among the Actinoids we have both simple individuals and 

 compound communities. But here again it is among the compound communities that 

 we find the higher organic combinations : for certainly the Madrepores, with their 

 twelve tentacles, alternately larger and smaller, are superior to the Astrjeoids, and 

 these again superior to the Actinioids ; and that these latter are the lowest will 

 hardly be doubted, if we consider the absence of solid deposits in them, and the 

 equally characteristic absence of horizontal floors between their radiating jaartitions. 

 It will be conceded also that the Fungidoi stand next above them, since they 

 have a large number of tentacles, like the Actinia^, and only transverse beams 

 extending from one radiating partition to the other, instead of continuous floors 

 as in the Astrseoids, which stand above them on that account, as well as on account 

 of their limited number of tentacles. The Madrepores, unquestionably, are the 

 highest among the Actinoids, since they not only present a limited number of 

 tentacles, but a number which is always constant, and, in addition to this, another 

 higher combination of structural features, arising from an alternation of larger and 

 smaller tentacles and a marked one-sidedness of their calycles. 



It is thus plain that the gradation among Actinoids, — that is, the higher and 

 higher rank they occupy when compared with one another, — stands in direct ratio 

 to their comj)lication and to their combination into communities, and in an inverse 

 ratio to their individual independence. The simple Polyps, such as Actiniae, are the 

 lowest; the Fungidoe, among which there are simjile types, such as the genus 

 Fungia proper, are next in rank ; the Astraeoids and allied famihes, which form 

 always compound communities, with a reduced but more definite number of tenta- 

 cles, come next; and the Madrepores, forming among the Actinoids the most compli- 

 cated communities, stand highest. 



