VI. Holothurioidea. 21 



For the physiological details of this action consult the original. - - The state of 

 contraction or distension of the animal has an important influence on the relations 

 of the nervous system, and this partly accounts for the varying descriptions 

 of it given by different authors. The nervous layer of the integument, which every- 

 where in the interradii adheres to the muscular layer beneath it, is separated 

 from it along each mid-radial line, so as to leave a continuous radial lacuna 

 in the partition between water-vessel and nerve. Superficially to this the nervous 

 layer is traversed by two channels, subneural and extraneural, hollowed out 

 in its connective tissue, and separated by a horizontal interlocular partition of the 

 same material. On the under side of this is the inner part of the radial nerve band, 

 Semper's n 2 , traversed by a median groove and roofing in the subneural canal. 

 This is lined by a layer of connective tissue cells, rounded under the nerve band, 

 into which they send supporting fibres, but flattened elsewhere. Above the inter- 

 locular partition is the thicker external nerve band (n 1 ), which floors the extra- 

 neural space and is traversed vertically by supporting fibrils of connective tissue 

 which rest on the partition below it. Other fibrils enter the nerve from the globular 

 cells which cover it, and the latter are closely packed along two lines, right and 

 left of the centre, so as to divide the nerve band into three nearly equal parts. The 

 inner nerve band (n 2 ), is unrepresented in other Echinoderms, while the outer one 

 n 1 ) corresponds to the inner part of the radial trunk in Dorocidaris. The intra- 

 neural and perineural spaces of this type are respectively extraneural and sub- 

 neural in Holothurians. The latter terminates blindly against the wall of the peri- 

 buccal sinus, while the median groove of the inner nerve band (n 2 ) deepens and splits 

 the nerve into two parts. These consist of a connective tissue network containing 

 nerve fibrils, and bend downwards towards the origins of the longitudinal muscles 

 in the calcareous ring, where they are lost. The interlocular partition terminates 

 in the wall of the peribuccal sinus round which is the annular extraneural space 

 with the nerve ring, formed entirely from (n 1 ), in its floor. The tentacular nerves 

 originate directly in the oral ring, and those of the tube- feet are likewise almost 

 entirely derived from the outer band -- (n 1 ) thinning out and disappearing. But the 

 lateral branches of the radial cords which enter the interradii are chiefly derived 

 from (n 2 ), and are accompanied by extensions of the extraneural space. TheHolo- 

 thurians differ from the Urchins in having not only the radial but also the inter- 

 radial portions of the nervous system enclosed in the connective tissue of the 

 middle zone, and its lacunar system represents the schizocoel cavities of Asterids. 

 The wall of the digestive tube contains a subepithelial lacuna from which 

 amoebocytes pass upwards between the filiform cells of the epithelium. They 

 have been mistaken for gland cells and possibly absorb nutritive substances di- 

 rectly. There are also amoebocytes in the middle layer of the gut, both imbedded 

 in its structureless substance and in its lacunae, which communicate by openings 

 in the muscular coat with the system of marginal lacunae. The pharyngeal wall 

 contains bundles of nerve fibrils, apparently identical with the subdivisions of the 

 internal nerve band of the rays (n 2 ). - The mesentery of Colochirus Lacazii is 

 in four divisions, dorsal, ventral, lateral, and an intermediate one depending on 

 the development of the intestinal loop. The dorsal mesentery is in two parts, 

 separated along the line of Vogt & Yung's problematic canal [see Bericht for 

 1886 Ech. p 12], which is the genital blood-lacuna. Its anterior part or ineso- 

 arium, related to the fore-gut, includes the water-tube and the genitalia; it 

 .epresents the oesophageal mesentery of Urchins, and the wall of the axial sinus 

 in Asterids. In other Dendrochirotae the intermediate mesentery is reduced to 

 an open meshwork of very delicate threads, while in the Aspidochirotae it is more 

 directly continous with the dorsal and lateral mesenteries. The amoebo- 



