412 THE VOYAGE OF H.M.S. CHALLENGER. 
suggested that the great development and calcification of this lamella would bring 
the ambulacral nerve into a position corresponding to that of the radial nerve-cords 
of Ophiurids and Echinids. Marshall’ has recently put forward a somewhat similar 
hypothesis, viz., that this lamella “probably represents the earliest stage in the 
process by which the nerve becomes detached from the epidermis and shifted inwards.” 
We know far too little, however, about the ontogeny of the Echinoderm nervous 
system to do more than speculate on this subject. According to Selenka and 
Ludwig the nervous system of Asterids and Holothurids is of ectodermic origin ; while 
Gotte’s observations lead to the conclusion that the ambulacral nerves of Crinoids are 
derived from the endoderm. Should this really be the case, there can be no difficulty in 
taking the same view respecting the axial cords.” But even then we get no clue to the 
morphology of the central capsule, as Marshall has conveniently called the fibrillar 
envelope of the chambered organ in which the axial cords originate. 
He remarks’ that “ Dr Carpenter’s observations lead to the belief that, at any rate in 
its present form, it is connected with the change from the pedunculate to the free- 
swimming condition ; and it is worthy of notice that the two actions with which it has 
been found to be specially concerned physiologically, z.e., the movements of swimming 
and of righting, are ones that the pedunculate form, from the very nature of things, can 
never exercise.” 
I cannot quite share Marshall’s belief in the relation between the central capsule and 
the change from the attached to the free mode of life. The only difference between the 
chambered organ of a Comatula and that of a Stalked Crinoid is the absence of any 
cirrus-vessels in connection with the latter ; for these come off from the peripheral vessels 
of the stem (Pl. XXIV. fig. 4; Pl. LXII.—cv), which are the downward extensions of the 
cavities of the chambered organ. But the central capsule or fibrillar envelope of these 
cavities, which in Comatula “is specially connected with the complex co-ordinated move- 
ments of swimming and of righting when inverted,” is equally present in all the Stalked 
Crinoids (Pl. VIIb. fig. 2; Pl. XXIV. figs. 6, 7; Pl. LVIII. figs. 1,3; Pl LXII°); and 
there can be no doubt that it controls the movements of flexion and extension of the arms. 
The latter of these is essential to the proper nutrition of the animal; and I can quite 
believe that the arms may also be used for swimming by those Pentacrinidee, such as 
Pentacrinus maclearanus, Pentacrinus alternicirrus, and Pentacrinus wyville-thomsoni, 
which have short stems terminating below at a nodal jomt (Pl. XVI. fig. 1; Pl. XIX. fig. 1). 
In all the Stalked Crinoids the central capsule is continued downwards into the stem as a 
sheath around the central vascular axis (Pl. VIIa. figs. 1, 2; Pl. XXIV. figs. 1-6; 
Pl. LXII.—ca), and it gives off branches which spread out towards the surface of the stem, 
1 Loe. cit., p. 546. 
* Whatever be the origin of these cords, they are essentially mesodermic in their distribution ; and it isin this sense 
that I have spoken of them in the text as constituting a mesodermic nervous system (p. 114). 
3 Loe. cit., p. 547. 
