VIII EGHINODERMATA— SENSORY ORGANS 465 



specific sensory organs (the so-called sensory buds) been found on the inner side of 

 the oral feelers (i.e. on the side turned to the mouth). 



It has further been conjectured that the highly developed sense of smell of many 

 Echinoderms is located in ambulacral appendages, and in the Asteroids it has been 

 localised especially in the terminal tentacles of the arms, above described. Decisive 

 experiments, however, have still to be made on this point. 



The innervation of the ambulacral appendages is as follows: The tube -feet 

 or tentacle nerves are always lateral branches of the radial nerve trunks of 

 the superficial system, these lateral branches being accompanied by ganglion 

 cells. When the base of the tube-foot or tentacle is reached the nerves have, 

 in the Echinoidea, an epithelial course ; but in the Ophiuroidea and Holo- 

 thiirioidea, they are subepithelial even within the foot or tentacle. In the Orinoidea 

 and Asteroidea, these nerves, like the superficial nervous system, are, as we should 

 expect, epithelial. 



The condition of the nerves within the appendages varies. 



In the Asteroidea and Orinoidea, no localised distinct tentacle or tube -foot 

 nerve can be distinguished, a layer of nerve fibres being developed within the 

 epithelium of the entire foot. In the OpMuroidea, Solothurioideu , and Echinoidea, 

 on the contrary, these nerves are distinct even in the feet (in the first two groups 

 they are subepithelial, and in the last epithelial), and their ramifications can be 

 clearly followed. 



When the nerve reaches the base of a tentacle in the Ophiuroidea, it forms a large 

 semilunar or semicircular ganglion encircling the base, before it ascends into the 

 tentacle (Fig. 372, p. 456). 



Round the terminal disc, in the tube-feet of the Echinoidea and Asteroidea, the 

 (epithelial) nerve tissue becomes thickened to form a nerve ring, from which the 

 nerve fibres (arranged as a whole radially) run inwards close to the epithelium of the 

 terminal disc. 



The way in which the nerve fibres end in the epithelium of the ambulacral 

 appendages is still a disputed question. According to one view, the nerve fibres are 

 connected with filiform sensory cells, which have been brought to light by means of 

 maceration. According to another view, no such cells are present, the nerve fibres 

 and epithelial cells being merely in contact. 



Special. — Any special terminal apparatus of the sensory nervous system has 

 rarely been observed in the ambulacral appendages. We have, however : — 



(a) The sensory buds on the oral tentacles of the Synaptidcc, already mentioned 

 above. These are arranged in two longitudinal rows along the inner side of each 

 tentacle. They are conical or papilla-like projections of the tentacle wall, with a 

 pit-like invagination at the tip. A nerve from below the surface of the cutis enters 

 the base of the pit, which consists of strongly ciliated sensory cells. It has already 

 been pointed out that olfactory or gustatory functions have been attributed to these 

 sensory buds. 



(6) The tentacles of the Orinoidea carry scattered sensory pajHllEe in the form of 

 distinct slender projections. Each papilla is composed of the fine processes of the 

 circle of sensory cells, which form its base. It is traversed by a shiny axial (muscle) 

 fibre, and carries at its freely projecting end three delicate, thin, immobile sensory 

 hairs (Fig. 356, 14, p. 413). 



(c) Similar sensory papillie are found on the tentacles of the Ophiuroid form 

 Ophiactis virens. 



{d) In certain species of the Ophiuroid genus Ophiothrix, the tentacles are 

 covered with circular rows of conical sensory papillfe. Each papilla seems to 

 consist of a bundle of long sensory cells, and carries freely projecting sensory hairs 

 (Fig. 375). 



VOL. II 2 H 



