REPORT ON THE CRINOIDEA-. 



413 



and probably form a subepidermic plexus (PI. Vila. fig. 1 ; PI. XXIV. fig. 2 — ca'). 

 Extensions of it also proceed into the cirri round the cirrus-vessels (PI. LXII., cv), and 

 give off" similar radiating branches. Jickeli's observations on Antedon rosacea show that 

 the movements of the cirri, like those of the arms, are dependent upon an influence pro- 

 ceeding from their axial cords ; ' and it is clear that if the central capsule and axial cords 

 of Comatula constitute its principal nervous system (as few will now deny), this must be 

 regarded as also extending throughout the whole stem of a Stalked .Crinoid, even though 

 it reach 70 feet in length, as in the fossil Extracrinus subangidaris. 



It may also be assumed with tolerable certainty that there was a similar neuro- 

 vascular axis in the stem of all the fossil Pelmatozoa, including the Blastoids. These had 

 no arms attached to the radials, as a Crinoid has. But if they had the same arrange- 

 ment of axial cords in the basals and radials as prevail in a Crinoid, which I see no reason 

 to doubt, it is unlikely that the radial cords, starting from the circular commissure 

 (fig. 20, A) should have ended in the forks of the radials which receive the distal ends of 

 the ambulacra. In the Mesozoic Phyllocrinus the radials have almost exactly the same 

 forked shape as those of a Pentremites ; l^ut the point where the limbs of each fork 

 separate is marked by an articular facet to which the arm was attached. 



In a Blastoid, however, the sinus between the two limbs of the radial is filled up by 

 the ambulacrum, which terminates in a more or less prominent lip at the same point in 

 the body of the radial as is occupied by the articular facet in 

 Phyllocrinus. That the axial cord in the radial of a Pentremites 

 (fig. 20, A) ended in this lip seems to me improbable ; and I 

 cannot help suspecting that it may here have become continuous 

 with the nerve of the ambulacrum. From what we know of the 

 ambulacral nerves of recent Crinoids, this is not likely per se to 

 have been related to any other movements but those of the 

 tentacles, even if these organs were present ; while it may 

 perhaps have been removed from the superficial epithelium, as 

 in an Urchin, and lodged within one of the canals in the lancet 

 plate. This would have brought it deep enough to be continuous 

 with the axial cord of the radial at the termination of the 

 ambulacrum. The movements of the so called pinnules of the 

 Blastoid would thus have been controlled by the central capsule, 

 instead of by the oral ring, which is not likely to have had 

 anything to do with them ; for the oral ring of a recent Crinoid 

 has absolutely no control over the movements of any part of the 

 calcareous skeleton ; and the jointed pinnules of a Blastoid cannot have remained 

 permanently closed over the ambulacra, as they are found in the fossils (fig. 20). 



' See the r|uotation from Agassiz, ante, p. 333. 



Fig. 20. — Diagi-am showiDg the 

 arrangeinent of tlie axial cords 

 in the calyx of a Pentremites, 

 supposing it to be the same as 

 in the calyx of a Crinoi J. The 

 pinnules are represented as 

 preserved on one ainbulacruni, 

 but not on the other. ^ , axial 

 cord of the ray; ai, primary 

 interradial cord" ; «'/■, one of the 

 converging radial cords which 

 result from its bifurcation; 

 B, basal; ceo, circular com- 

 missure ; /, interradial plate 

 {deltoid of Koemer) ; Ji, radial. 



