EC HINODERM A GENERAL DESCRIPTION 



Sense-organs are but slightly developed. They are tactile, 

 visual, and auditory or orienting. Tactile organs are furnished 

 by the ambulacral appendages, the spines, and the pedicellariae. 

 'The chief among the ambulacral appendages is the " terminal 

 tentacle," the unpaired end of the perradial water-canal, differ- 

 entiated only in Stelleroidea and Echinoidea (Fig. XXV. 1). In 

 Asteroids it is coated with columnar epithelium bearing long cilia 

 and innervated by the radial nerve; in Ophiuroids this nerve, 

 which is sub-epithelial in the arm, becomes epithelial in the 

 tentacle ; in Echinoidea the terminal tentacle is a sensory papilla 

 penetrating the pore in the " radial " or " ocular " plate of the 

 apical system. The remaining ambulacral appendages, the podia, 

 whether sucking feet, as in Echinoidea, Holothurians, and Asteroids, 

 or tentacles, as in Ophiuroids and Crinoids, are highly sensitive, and 

 sometimes have special developments. Thus, on the adoral side 

 of the oral tentacles of Synaptidae are two rows of papillae, of 

 which the tip is concave and ciliated ; these are called " sensory 

 buds," and supposed to be organs of smell or taste. Again, the 

 podia of Crinoids have small papillose projections, each papilla 

 armed with three stiff but fine hairs. Similar papillae, sometimes 

 more developed, occur in some Ophiuroids arid Echinoids. Spines 

 occur chiefly in Echinoids, less pro- 

 nounced in Stelleroids, and rarely in 

 Crinoids and Blastoids. Not all spines 

 are sensory. The smaller spines of Cidar- 

 oida, surrounding the larger spines, and 

 the main openings of the theca, are 

 covered with ciliated epithelium, and 

 bear tactile hairs at the tip ; the minute 

 spines (davulae) on the fascioles of Spat- 

 angidae (see p. 319) likewise have a 

 ciliated integument, probably with sen- 

 sory cells. The club-shaped spines of 

 some Ophiuroids are covered with a 

 glandular and sensory epithelium. Pedi- 

 cellariae occur in all Echinoids, some 

 Asteroids, and a few Ophiuroids; they are 

 small, forceps -like appendages derived 

 from spines (see p. 287). All are covered 

 with a glandular, sensory epithelium, 

 which in the "glandular pedicellariae" 

 of some Echinoids develops special tactile 

 prominences. Visual organs are known 

 only in all Asteroids, a few Echinoids, and 

 Synapta, but other (probably all) Echinoderms are sensitive to light, 

 owing, perhaps, to the action of the pigment-bearing amoebocytes. 



Fio. XXVI. 



A single cup of Fig. XXV. 4, 

 reconstructed from the evidence 

 of Sarasin. Outside is a ciliated 

 cuticle, covering the transparent 

 cellular cornea ; below is a refrac- 

 tive body (ref), possibly a vacuo- 

 late and multinucleate cell ; the 

 nuclei (mi) lie in strands of proto- 

 plasm ; outside the base of the 

 cup is a layer of anastomosing 

 pigment cells, which pass up from 

 the pigment layer below through 

 the ganglionated (gg) nervous 

 layer. 



