414 COMPARATIVE ANATOMY CHAP. 



(i) The endothelium of the body cavity. 



(k) The much diminished body cavity itself (enterocoel, 17 and 21). 



(/) The dorsal (apical) body wall, which in this connection is of no 

 further interest. 



Crinoidea (Fig. 356). On the section of the arm of a Crinoid, 

 proceeding from the oral to the apical side, we find : 



(a) The body epithelium covering the food groove. 



(6) Deep in this epithelium, the radial nerve trunk of the super- 

 ficial oral system (1). 



(c) Below the epithelium (not invariably present) a small schizo- 



coel canal (pseudohsemal canal (2). 



(d) The radial canal of the water vascular system (3). 



(e) At its two sides, the paired subepithelial longitudinal nerves of 



the arms (4). 



(/) The three radial sinuses; viz. two paired sinuses (5 and 11), 

 separated by a vertical septum (the so-called ventral or sub- 

 tentacular canals), and a third unpaired sinus (7), the 

 dorsal canal, separated from the first two by a horizontal 

 (transverse) septum. 



All these parts lie embedded in somewhat sparse connective tissue. 

 In the middle between them run : 



(g) Tiie narrow genital sinus (G), with the genital tube (rachis) 

 within it. 



(h) The skeletal ossicle of the arm, or (according to the plane of 

 the section) the apical and oral muscles and bands, uniting 

 the ossicles. 



(i) In the centre of the joint we find the section of the nerve canal 

 (axial canal) with the radial trunk of the apical nervous 

 system (8) which it encloses. 



The figure also shows the tentacles, and the nerves which connect 

 the paired radial nerves of the oral with the radial trunk of the apical 

 nervous systems. 



V. The Integument. 



The integument of the Echinodermata consists of (1) the uni- 

 laminar body epithelium which covers the whole body with its processes 

 and appendages, and (2) a strong subjacent connective tissue layer 

 (the eutis or eorium) of mesenchymatous origin, in which the various 

 skeletal structures develop. The cutis forms by far the largest part 

 of the body wall. Internally, it is either directly lined by the endo- 

 thelium of the body cavity or else is separated from the endothelium 

 by musculature (Holothuria, Asteroidea). 



(1) The body epithelium. () This is distinct from the subjacent cutis in the 

 Echinoidca, Asteroidea, many Holothurioidea, and on the oral surface of the disc and 

 arms of the Crinoidea ; also in the Euryalcc. 



In the OpJiiuroidca (excluding Euryalcc), and on the apical side of the disc and 



