466 



COMPARATIVE ANATOMY 



(c) Sensory papillre, sensory hairs, etc., liave also been observed on the various 

 sensory feet of tlie EcMnoidea. 



[On tlie polymorphism of the ambulacral appenda^^es, see the section on the 

 Water Vascidar System, D, p. 431. 



B. Nerve Endings in the Integument. 



A close plexus of nerve fil)res is developed within the body epithelium of the 

 Echinoidea and Asteroidea. This jilexus is more strongly developed, i.e. closer and 

 thicker at points which are specially sensitive to external stimuli, such as the 



fascicles of the Echinoidea, roiind 

 •W the bases of the pedicellarife, and on 



the gills (the so-called papulte) of 

 the Asteroidea, ahd at the bases of 

 the spines of the Echinoidea. 



[For the sensory prominences on 

 the pedicellarife, see p. 399.] 



In the Crinoidea and Oplii- 

 2croidea, sensory nerves continually 

 ramifying more and more finely, 

 run tlirough the (calcified) cutis to 

 the surface of the body. The 

 manner in which these nerve fibres 

 terminate is unknown. Investiga- 

 tion on this point is the more 

 difiicult as the ej^ithelium appears 

 to be hardly distinguishable from 

 the cutis. 



At the edges of the food grooves 

 (on the arms and the oral disc) of 

 the Crinoidea, alternating with the 

 trilobed tentacles, groups consisting 

 of five to six sensory cells with 

 delicate immobile hairs, occur. 



Among the Holothurioidxa, a 

 system of nerve fibres ramifying in 

 the cutis has been described in 

 Cucumaria. From these branches 

 run to the nests of epithelial cells 

 sunk below the surface, wdiich were 

 mentioned in connection with the 

 integument, p. 415. A similar 

 arrangement has been found in 

 other ActiiLopoda. 



In the Paractinopoda {Synapta, 

 Anapta) numerous scattered sensory or tactile papillre are found on the integument, 

 which, at such points, bulges out to form prominences. At the centre of such a 

 prominence a group of sensory cells forms the tactile papilla. A distinct nerve runs 

 from each papilla to a large tactile ganglion lying in the cutis beneath it. The 

 epithelial cells surrounding the papilla are differentiated into glandular cells 

 {Synapta inhaerens). 



Fin. 375.— Half Of a transverse section through 

 an ambulacral tentacle of Ophiothrix fragilis (ccnii- 

 billed froui figures by Hamann). 1, Body eintheliuni ; 

 2, sensory papillae ; 3, cuticular connective tissue ; 

 4, nerves to tlie sensory papillie ; 5, longitudinal mus- 

 culature ; 6, epithelium of tlie tentacle canal (") ; 

 8, tentacle nerve. 



