II. Pelmatozoa 9 



communicate in every segment of the arms by a branch which leaves the dorsal 

 nerve on the side opposite to that proceeding to the pinnule. The ventral system 

 is less developed in A. Eschrichti, the papillae of the tentacles receiving their 

 nerves from the ambulacral system. Indications of segmentation are visible in the 

 dorsal nerves, a group of unipolar cells occupying the ventral surface of the nerve 

 in the middle of each arm joint, and the fibrils of the paired dorsal and ventral 

 branches are connected by commissures. In A. carinata the dorsal nerve trunks 

 have a well marked neurilernma which is but slightly developed in other species. 

 It is thickest round the cirrus nerves, occupying the greater part of their section, 

 and consists chiefly of a fibrous network in the meshes of which lie fibres and 

 cells of connective tissue. It is thinner round the arm nerves, but sometimes ex- 

 tends right through them, so as to divide them into two lateral halves. In Penta- 

 crinus decorus the peripheral branches of the arm nerves bear very large cells, 

 which are occasionally collected into small ganglia. The ambulacral epithelium 

 of Crinoids, like that of other Echinoderms , consists of supporting cells and 

 sensory cells, both of which bear cilia. The basal prolongations of the latter have 

 small varicosities and may be followed into the nerve layer, which is purely epi- 

 thelial, its fibres lying between the cell bases. It forms no ring around the mouth, 

 but is continued directly downwards into the gullet, where it gradually thins out. 

 - The external epithelium is without a cuticle and often difficult to make 

 out, except by the cell nuclei. The connective tissue beneath it is looser near the 

 surface, but firmer towards the coelomic epithelium, which covers all the bands of 

 tissue that traverse the body cavity, together with the vessels, water-tubes etc. 

 The latter are not continuous with any of the vessels, as stated by Perrier, and 

 by Vogt & Yung [see Bericht for 1886 Ech. p 5, 6]; nor is this the case with the 

 canals proceeding from the water-pores, which open exclusively into more or less 

 restricted portions of the body cavity. The epithelial cells lining the ventral wall 

 of the water-vessels are prolonged into muscle fibres which lie parallel to the 

 axis of the vessel. The epithelium in the ciliated cups of the pinnules chiefly 

 consists of fusiform cells with their nuclei at different levels, but at the bases of 

 the cups the epithelium is flattened. - The ventral openings of the cavities of 

 the chambered organ lead into short canals which do not pass upwards into 

 the axial organ, but terminate blindly around it. The cirrus vessels all originate 

 in the chambers and the connective tissue partitions within them are derived from 

 the partitions between the chambers. - The subarnbulacral nerve-vessel 

 really exists . though it is denied by Vogt & Yung , but is merely a schizocoel 

 space, unconnected with the blood-lacunar system ; and the genital canal of the 

 arm, though communicating both with the coelom and with the exterior, is pro- 

 bably also a schizocoel. The blood-lacunar system is very highly developed, 

 definite channels existing in the bands of connective tissue which traverse the 

 coelom ; but they have no endothelial lining and are continued into the wall of 

 the gut, and the connective tissue sheath of the axial organ. Neither are they in 

 any way connected with either water-pores or water-tubes.- The axial organ 

 consists of numerous tubes imbedded in connective tissue which are lined by an 

 epithelium of cylindrical granular cells. They have no duct, but end blindly both 

 dorsally and ventrally. Towards the dorsal side they -become smaller and less 

 numerous, with a flatter epithelium, and form the central axis of the chambered 

 organ. - - The genital cord of the arm is enclosed in a genital tube, which is 

 itself suspended in the genital canal (schizocoel) by spindle cells. Both tubes and 

 cords are continued into the disk, where they form an irregular pentagon between 

 the perisome and the gut. The ova have no follicle epithelium and discharge 

 their polar globules before leaving the pinnules. - - The musculature of the 



