REPORT ON THE HOLOTHURIOIDEA. 129 



The Nervous System. 



The nervous system corresponds fully with that in other Holothurioidea. It consists 

 of a ring (PI. XLV. fig. 5), which lies superficial to the calcareous ring and to the 

 circular water-vessel, and which sends off five cords, which proceed along the middle 

 line of the longitudinal muscular bands, to the opposite extremity of the body. From 

 the five cords already mentioned branches proceed to the pedicels and processes, whde 

 the ring gives off nerves to the tentacles as well as to the mouth. There is no 

 difficulty in following the nerve-branches to the tops of the pedicels and tentacles, but 

 I have not been able to discover in what manner they terminate. It would seem 

 that they divide into fine filaments which are in relation to the more or less elongated 

 cells which are present in great numbers at the ends of the pedicels and tentacles. 



In all the Elasipoda there exists a very well-developed delicate peripheral plexus of 

 nerves (PL XLV. fig. 1), formed by the branching and interjunction of an indefinite 

 number of larger and finer threads or fibres, which are in connection with numerous 

 ganglionic cells with distinct nuclei, and are often produced into several processes or 

 threads (PL XLV. figs. 2, 3) ; even in the pedicels, tentacles, and dorsal processes such a 

 network is present. The large ambulacral nerves as well as their branches often con- 

 tain pigment, which is most obvious in Lcetmogone. Considering the more or less 

 macerated condition of the animals, any closer examination of the histological structure 

 of the nerves has been rendered impossible. What I have been able to distinguish 

 concerning it seems to confirm the correctness of the observations so carefully made 

 by Semper, 1 and Teuscher. 2 Plate XLIL, fig. 1, represents a transverse section of an 

 ambulacrum in Lcetmogone loyville-thomsoni showing the relative position of the radial 

 ambulacral vessel and the neural canal, &c. Leaving the nerves, I now purpose to treat 

 the sensory organs of the Elasipoda. 



There is much reason to beheve that the dorsal processes and appendages per- 

 form in a similar or higher degree than the tentacles the function of tactile organs. 

 Resembling the pedicels in structure, they differ from them by their position and 

 unusual length, size, and flexibility, as well as by their lack of any terminal sucker, and 

 they seem to be organs particularly suited to bring the animals into relation to 

 surrounding bodies. An unusual abundance of nerves radiating towards the exterior 

 layer of the perisoma is a special characteristic of the processes (PL XXXVII. fig. 8). 



In the Deimatidse and the Psychropotidse I have found only the sensory organs 

 just mentioned, but most representatives of the Elpidiidse possess, besides these, olfac- 

 tory organs in the form of auditory sacs. It is an ah-eady well known fact that 



1 Reisen im Archipel der Philipphien, ii., 1, Holothurien, Leipzig, 1868. 



3 Beitrage zur Anatoniie der Ecliinodermeu (Jenaische Zeitschrift fur Naturwiss., x., Jena, 1876). 



(ZOOL. CHALL. EXP. PAET XIII. 1881.) N 17 



