MONOGRAPH OF THE EXISTING CRINOIDS. 333 



parallel fibrillse passing downward along the sides of the funnel instead of passing 

 concentrically around it. 



Wherever, as on the oral pinnules and distal extensions of the arms and 

 pinnules in Heliometra glacialis and other types, on the genital pinnules of certain 

 of the Charitometridse, and on whole arms, and even whole post-radial series in 

 certain of the Comasteridse, the tentacles and ambulacral epithelium are absent, 

 the epithelial nerves are also absent. 



The fibrillse composing the epithelial nerves are exactly like the fibrillae of 

 the dorsal and ventral nervous systems, and may be at once differentiated from 

 the connective tissue elements by their strong refractive power. Scattered among 

 them are multipolar and bipolar ganglion cells of which the cytoplasm is often 

 scarcely perceptible. 



The connective tissue lamella described by Ludwig and others between this 

 nerve layer and the epithelium of the ambulacral grooves appears to be the 

 processes of the supporting cells. 



P. H. Carpenter believed that the epithelial nerve band was united with a 

 fibrillar plexus from the axial cords which he traced into the connective tissue 

 of the perisome forming the ventral surface of the disk ; but at present the opinion 

 is that there is no connection between the epithelial and either division of the 

 mesodermal nervous systems. 



The epithelial nerves appear to be identical in all the comatulids which up 

 to the present have been studied. 



PHYSIOLOGY OF THE CBINOIDAL NERVOUS SYSTEM. 



In order to determine the functions of the different portions of the nervous 

 system of the comatulids a considerable amount of experimental work has been 

 done by W. B. Carpenter, Perrier, Jickeli, and A. Milnes Marshall, especially 

 by the first and last. 



The results of this work have been thus summarized by Professor Marshall : 



All parts of the surface of the animal are sensitive, but in very unequal 

 degrees. Irritation of the dorsal surface of the calyx causes only slight move- 

 ments of the arms, unless the irritation is severe or prolonged. Irritation of the 

 dorsal or lateral surfaces of the arms, where the integument is very thin, causes 

 flexion of the arms, with extension of the pinnules close to the irritated spot. 

 The response is usually prompt, but the movement is only slight. Prolonged or 

 more violent irritation causes exaggeration of the movement, together with approxi- 

 mation of the adjacent arms toward the irritated arm as though to remove the 

 source of irritation, and in some cases active movement of the whole animal in a 

 direction away from the irritated arm. 



Irritation of the ventral surface of the disk between the ambulacral grooves 

 causes movements of the arms, but not nearly such active movements as when 

 the oral pinnules are touched. 



The epithelium of the ambulacral grooves is extraordinarily and exceptionally 

 sensitive, and the response to stimuli is very definite. The slightest irritation 



