410 THE VOYAGE OF H.M.S. CHALLENGER. 
ments which have led him to the following conclusions :—‘ 1. The central capsule and 
its prolongations, the axial cords and their branches, constitute the main nervous system 
of Antedon. 2. The central capsule is specially connected with the complex co-ordinated 
movements of swimming and of righting when inverted. 3. The axial cords act as both 
afferent and efferent nerves. 4. The subepithelial bands are probably also nerves, but 
their exact function, probably a special and subordinate one in connection with the 
ambulacral tentacles and epithelium, is not yet ascertained.” 
These conclusions are the result of a long series of experimental investigations, in 
which Dr. Carpenter’s fundamental observations were repeated.and largely extended. 
Marshall’s paper concludes with a valuable discussion of the morphological aspect of 
his results. Starting from the generally accepted doctrine that the Asterids are the most 
primitive group of the Echinoderms, he shows how this character is well illustrated by 
their nervous system. Hamann’s observations have demonstrated that this “ is in the form 
of a continuous nerve-sheath enclosing the whole body, and directly continuous with the 
external epidermis of which it forms the deepest layer. This nerve-sheath is thickened 
at certain places, notably along the ambulacral grooves, where it forms the five radial or 
ambulacral nerves.” Marshall points out that the analogies of the Coelenterates, 
Cheetognatha, and Palzeonemertines all tend to show the primitive nature of the Asterid 
nervous system.' There is no difficulty about the relation of the nervous system of the 
remaining Echinozoa to that of the Asterids; but the case is different with the Crinoids, 
on account of the antambulacral position of their principal nerve centre and its radiating 
extensions. Marshall, however, is inclined to consider them as ‘descended from forms 
which agreed with the recent Asterids in possessing a complete nerve-sheath (though 
possibly very unlike Asterids in other respects) ;” and he is therefore “ disposed to regard 
the antambulacral nervous system of a Crinoid, 2.e., the central capsule and axial cords 
with their branches, as being derived from the antambulacral part of the primitive nerve- 
sheath, and not as an entirely new set of structures possessed by no other Echinoderms.” 
He endeavours to show that the relations of the axial cords which lie in grooves on 
the surface of the radials of the Pentacrinoid larva (a permanent condition in some 
Paleeocrinoids) are ‘“‘ very similar to those of the ambulacral nerves of an adult Ophiurid 
or Echinid, and as the latter have certainly acquired their adult condition by becoming 
detached from the epidermis and shifting inwards, so also may the same process be 
supposed to have occurred in the Crinoid.” Too much weight must not be laid upon 
this point, however, for the supposed inward movement of the radial nerves of an 
Ophiurid or Echinid would be from the outer or ambulacral surface of the plate towards 
the inner one, 2.e., that next to the body-cavity. 
On the other hand, in the developing Crinoidal calyx the axial cords are at first on the 
walls of the body-cavity, which are formed by the nner surfaces of the radials; but they 
1 Compare Chapter VI. pp. 113, 115. | 
