30 THE VOYAGE OF H.M.S. CHALLENGER. 
furnished by the Liassic Evtracrinus, the stem of Extracrinus subangularis reaching a 
length of 50 to 70 feet. But even as regards Democrinus I cannot admit that the 
dimensions of the stem are so much greater than those of the arms. This may indeed be 
the case in Perrier’s three specimens, of which “deux sont totalement dépourvus de bras ; 
le troisiéme n’en présente que des restes trés courts, d’aprés lesquels il est aisé de voir 
que les bras devaient étre extreémement peu développés.” But in the Carribbean examples 
of Rhizocrinus rawsont the longest stem (180 mm.) contains sixty-eight joints above the 
root, while there are five arms, each consisting of about eighty joints. Nearly half of 
these bear pinnules, so that even if the radicular part of the stem is taken into account, 
the superior dimensions would seem to be on the side of the arms rather than on that of 
the stem, which Perrier considers to represent five or six times the volume of the calyx 
and arms together. 
It is likely enough that this may have been true in his three specimens of Demo- 
erinus, which had lost the whole or greater part of their arms, owing to fracture at the 
syzygies, as is only too often the case with both species of Rhizocrinus. But when a 
tolerably perfect individual is obtained the arms are found to be considerably more than 
“extremement peu développés,” as was so easily inferred by Perrier upon totally 
insufficient evidence. He goes on to say, “ Alors méme qu’ils ne vivraient pas en colonie, 
le volume considérable de leurs racines ramifiées, la ressemblance de ces racines avec les 
bras qui surmontent le calice et dont elles sont probablement homologues, suffisent 
i démontrer que la disposition arborescente des parties, préface en quelque sorte de la 
symmétrie radiaire, n’est pas plus étrangére au type des Kchinodermes qu’au type des 
Ceelentérés.” 
The relationship of the Echinoderms to the Ccelenterates need not be discussed here ; 
but the resemblance and “probable homology” which Prof. Perrier sees between the 
arms and the root of a Crinoid appear to me to be forced in the extreme. The arms 
are merely extensions of the body, containing the same nerves, vessels, and body-cavity 
as are found in the calyx, together with the fully developed genital glands which are 
usually sterile in the body. But the branches of the root have the same structure as 
the stem, as remarked by Perrier himself; and this is very different from that of the 
cup and arms. It is true that the rootlets, like the arms, are traversed by axial cords 
which are connected with the fibrillar envelope of the chambered organ; but there the 
resemblance ends. They support no soft parts as the joints of the arms and pinnules do; 
and being formed entirely on the right antimer are totally devoid of any of the ambu- 
lacral structures which are so important in the morphology of the arms. If the term 
“homologous” is to be employed for a mere superficial resemblance of this kind, a new 
word must be introduced to denote community of origin and morphological similarity. 
One might almost as reasonably say that the quills on the back of a porcupine are 
homologous with its limbs. 
