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 ejaidermis 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, 

 Chsetognatha, and Palseonemertines 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, i.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 

 Palgeocrinoids) are " very similar to those of the aml)ulacral 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 supjaosed 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, i.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 inner surfaces of the radials ; but they 



1 Compare Chapter VL pp. 113, 115. 



