23 



fig. 29 ; PI. IV., tig. 41, f/i. vs.) divide into three to form 

 the cavities of the triad groups of tentacles which spring 

 from a common point in the centre of the hippets which 

 form the margins of the ambulacral groove. Tentacles 

 and tentacular vessels are wanting in the oral pinnules. 

 In its main features the histological structure of these 

 tentacles agrees with that of the oral ones (PI. IV., fig. 47, 

 or. in.). Passing from within outwards there is (1) an 

 endothelium of rounded cells {coe. en.) lining the lumen of 

 the tentacle and resting upon (2) a thin but well-defined 

 layer of longitudinally disposed muscular fibres (l.m.f.) 

 with which the epithelial cells are said to be connected ; 

 (3) a layer of connective tissue, scarcely recognisable in 

 the pinnular tentacles ; (4 j fibrils from the sub-epithelial 

 nerve band {sh. nv. f .) (on the inner face of the tentacle 

 only); (5) the external epithelium (ectoderm = ef<.). In 

 the case of the pinnular tentacles, however, the cells of 

 the external epithelium are grouped together to form stiff 

 projecting sensory papilla? (PL III., figs. 30 and 31, 

 sn. 'Pl^-)- 



The water-tubes (stone canals of authors), to the 

 number of about thirty in each interradius, open into the 

 circum-oral vessel along its lower side (PI. IV., fig. 47 ; 

 PI. v., fig. 52, w.t.). The slightly expanded free ends of 

 the tubes depend and open into the peripheral portion of 

 the ccelom. Their lumina are lined by columnar cells 

 which are ciliated and have large oval nuclei which stain 

 deeply (fig. 47). The ciliated funnels (PI. II., fig. 22; 

 PI. v., fig. 52, cl. fn.) are scattered all over the tegmen 

 calycis to its extreme edge between the bases of the 

 arms, and are especially numerous along the borders of 

 the ambulacral grooves. Their slightly raised external 

 apertures, the water pores, are visible under a good hand 

 lens. Passing from without inwards the diameter of the 



