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



between the surrounding pigment bodies, and ramify in the small spaces left between 

 the petaloid areas that are occupied by the five interradial ligaments (PI. XXIV. fig. 1 ; 

 fig. 2, ca'). These extensions towards the surface of the stem eventually become so fine 

 that I have been unable to trace them in the somewhat thick sections with which I have 

 been obliged to content myself, owing to the large size and the toughness of the stem 

 ligaments. But from what I have seen in the arms, and more especially in the pinnules, 

 I have very little doubt that the ultimate subdivisions of these branches are in connection 

 with a subepidermic plexus. I have found similar branches in the cirri. 



The occurrence of this tissue around the axial vessels of the stem and cirri has been 

 employed by Ludwig as an argument against its nervous nature on account of the 

 absence of muscles in these organs.^ " Wozu also ein dieselben in ihrer ganzen Lange 

 durchziehender (motorischer) Nervenstrang?" The same argument might be employed 

 with respect to the extension of branches from the axial cords within the calyx up into 

 the anambulacral plates on the sides and ventral surface of the disk of Pentacrimis 

 decorus (PL LIX. figs. 2-4, ad). But their presence is readily understood if we consider 

 them as sensory nerves establishing a communication between an ectodermic plexus and 

 the axial cords of the rays, which all commence in the envelope of the chambered organ 

 situated within the calyx. It is however in the arms and pinnules that the lateral 

 extensions of the axial cords are most evident (PI. LXI. fig. 6), and the inadequacy of 

 Ludwig's theory as to their natiire is strikingly manifest. 



The species in which I have found these branches best develo^^ed are Bathycmius 

 aldrichianus, Pentacrinus decorus, Antedon eschrichti, Actihometra ixtrvicirra, and 

 Actinometra nigra. But they may also be met with in less abundance in the small 

 arms and pinnules of Antedon rosacea, and I cannot understand how they escaped the 

 notice of Greefi", Teuscher, and Ludwig. 



The lateral branches from the central fibrillar axis in the stem of Bathjcrinus 

 aldrichianus have been already mentioned (PI. Vila, fig. 1, ca'). The axial cords within 

 the rays and arms have a similar extensive distribution. Numerous branches proceed 

 outwards from them into the calcareous substance of the successive joints, as is shown in 

 the second and axilliary radials, which form an important part of the cup enclosing the 

 visceral mass (PL Vllb. figs. 6, 7, a'). The arms become free higher up, however, and 

 the deep median groove in the ventral surface of the skeleton receives the ambulacrum 

 with its armature of covering plates (PL VII. fig. 8). The axial cord gives off a large 

 branch on each side which proceeds upwards, subdividing freely as it goes. The ultimate 

 branches, many of which have bipolar cells intercalated in their course, extend right up 

 to near the top of the side-walls of the arm-groove, where they become so small that I 

 have been unable to trace them further (PL Villa, figs. 4, 5, a'). I have occasionally 

 seen a cell with three processes instead of two, but these are rare. The ambulacral nerve 



1 Crinoideen, loc. cit., p. 335. 



