466 



COMPARATIVE ANATOMY 



CHAP. 



(e) Sensory papillse, sensorj' hairs, etc., have also been observed on the various 

 sensory feet of the Echinoidea. 



[On the polymorphism of the ambulacral appendages, see the section on the 

 Water Vascular System, D, p. 431. 



B. Nerve Endings iu the Integument. 



A close plexus of nerve fibres is developed within the body epithelium of the 

 EcMnoidca and Asteroidea. This plexus 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, round 

 VjA the bases of the pedicellariie, and on 



the gills (the so-called papulse) of 

 the Asteroidea, and at the bases of 

 the spines of the Echinoidea. 



[For the sensory prominences on 

 the pedicellarife, see p. 399.] 



In the Crinoidea and Ophi- 

 uroidea, sensory nerves continually 

 ramifying more and more finely, 

 run through 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 

 diflScult as the epithelium 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 Holotlnirioidea, a 

 system of nerve fibres ramifying in 

 the cutis has been described in 

 Cuciiinaria. From these branches 

 run to the nests of epithelial cells 

 sunk below the surface, which were 

 mentioned in connection with the 

 integument, p. 415. A similar 

 arrangement has been found in 

 other Artinojpoda. 



In the Paractinoimda (Synapta, 

 Anapta) numerous scattered sensory or tactile papillie 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 inhaercns). 



Fig. 3TS.— Half of a transverse section througli 

 an ambulacral tentacle of Ophlothrlx fragills (coin- 

 bined from figures by Hamann). 1, Body epithelium ; 

 2, sensory papilli"e ; 3, cuticular connective tissue ; 

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

 culature ; 6, epithelium of the tentacle canal (7) ; 

 8, tentacle ner^'e. 



