78 
The interest attached to these curious specimens is much more 
than that of curiosity, this remarkable development of pinnules into 
arms having a very distinet bearing on the question of the mor- 
phological value of pinnules. 
A. H. Clark, in his „Monograph of existing Crinoids" 1 ) has 
pronounced some very startling ideas as to the morphological value 
of pinnules, ideas which are direetly connected with his theory that 
the ancestors of Crinoids are — the barnacles. 
The alleged derivation of Crinoids (and Echinoderms upon the 
whole) from Arthropods, of course, makes it necessary to seek some 
structure that might be regarded as the homologon of the Arthropod 
appendages. Clark finds such structures in the pinnules and the 
cirri, stating (Op. cit., p. 274) that „it is probable that the pin¬ 
nules and the cirri represent the original type of Crinoid append- 
age, and these appendages were arranged in five pairs, the two 
components of each pair being, so to speak, back to back; but 
both the pinnules and the cirri have become enormously redupl- 
icated, while in addition the former have come to lie along either 
side of long body processes of subsequent development". Also the 
elongate marginal cirri of some Comatulids, like Heliometrct, Pro- 
machocrinus etc. are regarded as a kind of tactile organs „dis¬ 
tantly suggesting the antennæ of arthropods". 
This interpretation of pinnules and cirri as the homologon of 
Arthropod appendages has further led to the assertion that „pin¬ 
nules beyond the second segment are merely elongated tentac.ular 
processes in which a skeleton is formed as needed", as are also 
the cirri „a long tentacular structure with no phylogenetic history" 
(Op. cit. p. 272). 
The development of pinnules into arms of exaetly the same 
structure as the normal arms in the specimens of Antedon petasus 
here described, decisively proves that the pinnule joints — also 
those beyond the second — have the same fundamental value as 
the brachials and necessarilv leads to the conclusion that the pin¬ 
nules morphologically represent arms; they are on physiological 
grounds reduced to organs specially adapted for generative, nutrit- 
ive and respiratory funetions, but retain a latent, potential power 
*) Buil. U. S. National Museum. 8.2 I. 1915. 
