GENERAL PART. 81 



The "long tentacular structure, with no phylogenetic history, which 

 forms within itself a series of skeletal braces as necessity requires " does not 

 agree with embryological facts; this applies to the pinnules as well as to the 

 cirri, which are structures of quite different morphological value, by no 

 means "exactly comparable." This is an imaginary construction grown 

 out of the necessity of finding a support for the astonishing idea held by 

 A. H. Clark that Crinoids (and Echinoderms upon the whole) are derived 

 from the barnacles. In the way quoted the pinnules and cirri are made to 

 represent the original type of Crinoidal appendage, the required homologon 

 of Arthropod legs, originally arranged in five pairs, "the two components 

 of each pair being, so to speak, back to back," and only later on, in the course 

 of development, "enormously reduplicated." 



A most remarkable and quite decisive corroboration of the view regarding 

 the morphological value of pinnules held by the present author (and, doubt- 

 less, by the great majority of specialists in Echinoderms) is afforded by the 

 curious specimens of Antedon petasus described in the author's paper, "Notes 

 on Some Scandinavian Echinoderms." 70 In these specimens some of the 

 pinnules (in some cases the first oral, in others the third pinnule) have 

 developed into true arms of exactly the same structure as normal arms and 

 carrying pinnules in the usual arrangement. These facts necessarily lead 

 to the conclusion that the pinnules morphologically have the value of arms, 

 but are on physiological grounds reduced to organs especially adapted for- gen- 

 erative, nutritive, and respiratory functions (more rarely for attachment, as in 

 the case of Comatulella brachiolata,'' 1 on which Clark lays so much stress), 

 but always retaining a latent, potential power of developing in the same way as 

 the normal arms. For the cirri such development is out of the question, 

 alone for the reason that they have no relation whatever to the water 

 vascular system, as they are upon the whole of quite another anatomical 

 structure than the pinnules. 



This result, as regards the morphological value of the pinnules, evidently 

 lends support to the assertion of Clark that each brachial is fundamentally 

 an axillary, although ontogenetically it does not originate as such. Also, 

 the pinnule joints, not the two basal ones alone, have fundamentally the 

 same morphological value, and as appears from the specimens of Antedon 

 petasus referred to any of them may, in fact, assume the shape and function 

 of an axillary. The instance of a pinnule bifurcating at the fourth joint 

 observed in Isometra vivipara 72 likewise bears testimony of the capacity of 

 any pinnule joint of transforming into an axillary, which could, of course, 



Vid. Medd. Dansk Naturhist. Foren. vol. 72, 1920. 



71 No information is given by Clark about the anatomical structure of the remarkably transformed 

 pinnules of Comatulella brachiolata; but there can hardly be any doubt but that they will prove to agree 

 essentially with the normal pinnules, and not with the cirri, both in regard to anatomical structure and 

 development. 



n Th. Morteusen. The Crinoida of the Swedish Antarctic Expedition. Wiss.-Ergebn. d. Schwed. 

 Siidpolar Expedition, 1901-1903, vol. vi, 1918, p. 13. 



