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



suwo-ested -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 Ophinrids and Echinids. Marshall^ has recently put forward a somewhat similar 

 hjqiothesis, 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, i.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 difi'erence 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 ofi" from the peripheral vessels 

 of the stem (PI. XXIV. fig. 4 ; PI. 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 eqiially present in all the Stalked 

 Crinoids (PL Vllb. fig. 2 ; PL XXIV. figs. 6, 7 ; PL LVIII. figs. 1,3; PL LXIL); 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 Pentacrinidfe, such as 

 Pentacrimis maclearaniis, Pentacrinus alternicirrus, and Pentacrinus ivyviUe-thomsoni, 

 which have short stems terminating below at a nodal joint (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 Vila. 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 Loc. cit., p. 546. 



- Whatever be the origin of these cor.ls, they are essentially mesodermic in their distribution ; and it is in tliis sense 

 that I have spoken of them in the text as constituting a mesodermic nervous syskm (p. 114). 

 ^ Loc. cit, p. 547. 



