30 THE VOYAGE OF H.M.S. CHALLENGER. 



furnished by the Liassic Uxtracrinus, the stem of Extracrinus siibangularis reaching a 

 leno-th 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 ddpourvus de bras ; 

 le troisieme n'en preseute c{ue des restes tres courts, d'apres lesquels il est aise de voir 

 que les bras devaient etre extremement peu developpes." But in the Carribbean examples 

 oi Rhizocrinus rawsoni 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- 

 crinus, 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 developpes," as was so easily inferred by Perrier upon totally 

 insufiicient evidence. He goes on to say, " Alors meme cpi'ils ne vivraient pas en colonic, 

 le volume considerable de leurs racines ramifiees, la ressemblance de ces racines avec les 

 bras qui surmontent le calice et dont elles sont probablement homologues, sufiisent 

 a ddmontrer cpie la disposition arborescente des parties, preface en cpielque sorte de la 

 symmetric radiaire, n'est pas plus ^trangere au type des Echinodermes cp:'au type des 

 Coelenterds." 



The relationship of the Echiuoderms to the Coelenterates 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 difiereut 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. 



