902 SUMMARY OF CUERENT RESEARCHES RELATING TO 



words, the axial cord conveys impulses centripetally ; and it appears 

 also to be the sole means of afferent communication. As to their 

 motor functions the results are similar, division of the axial cord 

 destroying motor communications. The two arms of each pair are 

 connected with each other by a transverse commissure. 



Tlie Sub-epithelial Bands appear to be nerves, and have probably a 

 special and subordinate function in connection with the ambulacral 

 tentacles and epithelium. 



The author concludes with some observations on the morphology 

 of the nervous system of Crinoids. He considers that the sub- 

 epithelial bands are homologous with the radial nerve-band of an 

 Asterid, and he looks upon the antambulacral nervous system (i. e. 

 the central capsule, and axial cords with their branches) as being 

 derived from the antambulacral part of the primitive nerve-sheath 

 which invested the body, and not as an entirely new set of structures. 

 The external and internal plexuses of Echinus, with their connecting 

 fibres in the substance of the calcareous test, offer us an arrangement 

 not altogether unlike that of the Crinoid. The difference, it must 

 be remembered, between Crinoids and the rest of the Echinoderms is 

 very great ; not only is the Crinoid condition primitive, it is also 

 highly specialized. 



Nervous System of Crinoidea.* — Dr. W. B. Carpenter recapitu- 

 lates the history of his inquiry into the nervous system of Crinoids, 

 and indicates the points in which the histology and anatomical dis- 

 tribution of the fibres, which he has thought to be nervous, sujiport 

 his view. The theoretical homology of the relations between Crinoid 

 and other Echinoderms is opposed by such facts as : 



1. The absence of any branches from the sub-ambulacral nerves 

 to the muscular apparatus of the Crinoidea generally. 



2. The absence of sub-ambulacral nerves from those pinnules of 

 Antedon which are most distinguished by their sensory endowments. 



3. The absence of the same nerves from a large proportion of the 

 arms of Actinometra, which, nevertheless, take their full share in the 

 co-ordinated swimming movements of these animals. 



4. The continued performance of these movements by Antedons 

 from which the whole visceral mass, including the oral ring, has been 

 removed, and by arms whose sub-ambulacral nerves have been cut 

 near their base. 



A point of great interest is the existence of a definite nervous 

 system with very little histological differentiation ; there is no 

 definite distinction between ganglionic centres and nerve-trunks; 

 almost every part of the apparatus is, probably, capable of originating 

 as well as of conducting. That the axial cords are not mere con- 

 ductors seems to be proved by the performance of active spontaneous 

 movements by arms detached several days before from the body. 



Dr. Carpenter refers to the views of his son and of Prof. Perrier 

 who have supported his views from the anatomical side, and to the 

 physiological experiments of Prof. Marshall and Dr. Jickeli. 



* Proc. Koy. Soc, xxxvii. (1884) pp. 67-76. 



