310 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
the following layers may be distinguished :—(1) an epithe- 
lium (ep.) composed of long fusiform cells; (2) a layer of 
connective tissue (c.t.l.) containing, in the inferior spiral, 
an extensive system of blood lacune; (3) a thin layer of 
muscular fibres (m.f.l.); (4) an outer and very delicate 
layer of connective tissue; (5) the ciliated epithelium of 
the ccelom (e.b.c.). A delicate cuticle (cw.) invests the 
epithelium of the gut, which is also ciliated. In the 
cesophagus and, to a much larger extent, in the inferior 
spiral, the epithelium contains numbers of gland cells 
(fig. 34, gl.c.). The relative thickness of the epithelial 
and connective tissue layers varies in different parts of the 
intestinal tract. The minute structure of the siphon is 
the same as that of the intestine. Its function has not 
been determined, but a suggestion has been made that it 
may subserve intestinal respiration. 
The Echinus is probably wholly carnivorous; and the 
frequent occurrence, in the intestine, of fragments of the 
shells of barnacles and serpulid worms attests the de- 
structive power of the alveolar teeth. 
WATER-VASCULAR SYSTEM. 
The water-vascular system consists of tubular canals, 
which communicate, on the one hand, with the exterior 
through the pores of the madreporite; and, on the other 
hand, with the cavities of the tube-feet and their ampulle. 
The madreporite (Pl. I1., fig. 1; Pl. III1., fig. 24, mad.) is 
traversed by a large number of minute pores (m-.p.), which 
converge towards and open into a cavity, the madreporic 
ampulla (mad.amp.), which lies beneath it. The external 
surface of the madreporite is covered by ectoderm cells 
like those which clothe the general surface of the test; 
but at the external openings of the pores the character of 
the cells suddenly changes, and the upper fourth of the 
