10 Echinoderma. 



cous secretion, and are restored by growth from the inner ends which retain the 

 nuclei. Each gland is enclosed in a layer of muscular fibres with fine longitudinal 

 striation, and the whole is imbedded in connective tissue. Like Duncan, H. regards 

 these organs as modified pedicellariae owing to their occasionally containing atro- 

 phied calcareous valves [see Bericht for 1886 Ech. p 11]. The smaller spines of 

 D. papillata have basal swellings which are composed of ciliated gland cells with 

 strongly refractile granular contents and flagellate epithelial cells connected with 

 nerve fibrils. In S. granularis and C. longispinus the spines have a basal nerve 

 ring, situated between the outer epithelium and the inner muscular layer, and 

 containing both motor and sensory fibres. - - As in the Holothurians the radial 

 nerves of Urchins have descended into the cutis, carrying with them the covering 

 or ambulacral epithelium with its supporting cells. Among the downward processes 

 of the latter are situated the nerve fibrils and intercalated ganglion cells, the whole 

 nerve having a schizocoel space on either side of it. It pierces the substance of 

 the radial (miscalled ocular) plate, where it loses the covering epithelium and the 

 schizocoel spaces, and its fibrils come into connection with the inner ends of large 

 cells forming a swelling of the epidermis above the blind end of the water vessel ; 

 but there is no extensile tentacle nor eye spot, though this swelling is homologous 

 with that at the end of the terminal tentacle of the starfish arm. The nerve of 

 each tube-foot gives off numerous branches at its base, which form a subepidermic 

 plexus, supplying spines, sphaeridia and pedicellariae. The ambulacral tube- 

 feet of an Urchin are not all of the same structure as stated by Koliler, but fall 

 into 3 classes. 1. The tactile feet on the back and sides of the test have no 

 sucking disc, but there is a projection or nerve-plate just below the pointed end, 

 and the terminal epithelial cells are not ciliated but bear short stiff processes as 

 in the ten buccal feet. The lower part of the cavity of the foot is crossed by inter- 

 lacing muscular fibrils. 2. The buccal feet have an elliptical or biscuitshaped 

 sucking disc, the epithelium of which chiefly consists of sensory cells connected 

 with the fibrils of the nervous prominence beneath it. 3. The sucking feet have 

 a circular disc the margin of which bears a specially nervous epithelium connected 

 by two large trunks to the nerve ring below it. Both marginal and central parts of 

 the disc contain sensory cells and also supporting cells, the long basal processes 

 of which are continuous with connective tissue fibres that pass through the open- 

 ings in the rosette and form a thick layer of longitudinal fibres. Between this 

 and the inner epithelium is a layer of circular connective tissue fibres , hitherto 

 regarded as muscular. - - Two branches proceed from the oral nerve ring and 

 enter the epithelium of the gullet, bundles of nerve fibrils lying among the basal 

 processes of the long epithelium cells. These also occur in other parts ,of the 

 digestive tube. Round the base of each sphaeridium is a ring like fold, chiefly 

 consisting of sensory epithelium cells which bear flagella and are probably con- 

 nected with the fibrils of the subjacent nerve ring. These facts indicate that the 

 sphaeridia are modified spines. The tactile cushions on the gernmiform pedicel- 

 lariae of S. are covered with a high epithelium, prominences of which bear stiff 

 hairs. The sensory cells are connected with fibrils of the subjacent nerve band. 

 This is traversed by the long basal processes of the supporting cells which pass into 

 the fibrils of the basement membrane beneath. Essentially the same characters 

 appear in both the tactile cushions on the pedicellariae of E.acutus. --The ciliated 

 pore-canals of the rnadreporite communicate with no other cavities than that of the 

 water- tube, the interior of which is smooth without any foldings. Most of its 

 epithelium consists of high cells with fusiform nuclei, each bearing a long cilium. 

 But in S. granularis the part of the water-tube next the axial organ bears cubical 

 cells with globular nuclei ; these are perhaps ciliated and so cause an outward 



