12 H. M. WOODCOCK. 



tlieu a delicate muscular layer {in. L), and, externally, a 

 single layer of coelomic epithelium (c. ep.). 



The wall of the complicated " rete mirabile,'^ on the other 

 l)and, always appears yellow to yellowish-brown and opaque, 

 and in section (fig. 8) is much thicker and of a firmer texture, 

 with a corresponding reduction in the lumen. The loose, 

 spongy tissue is less developed, and only forms a thin layer 

 most internally (b-j). c). Next comes the layer of muscle- 

 fibres {ill. I.), and, finally, making up nearly the whole thick- 

 ness of the wall, a great development of ccclomic epithelium 

 (c. 62).).^ 



Relation of the parasites to the blood-vessels. — 

 To return to our parasites. As they grow they are carried 

 about passively (for they lack all power of movement, see 

 below, p. 19) along with the blood-stream, and at last pass 

 into the larger thin-walled vessels — the distributing part. 

 Obviously it is much easier for them to evaginate the walls 

 here than it would be if they remained in the " rete mirabile"; 

 in fact, I never once saw a cyst attached to this latter. A 

 curious place of occurrence of the evaginated cysts is in 

 connection with the membrane which stretches from the ring- 

 canals to the ossicles in the body-wall and in which run the 

 radial canals. Lying in this membrane are numerous vascular 

 cross-connections between the radial vessels, and to these the 

 Gregarines are often attached in considerable numbers, having 

 evaginated their walls. 



I never noticed any cysts free in the body-cavity of the 

 Holothurian, a point in which C. irregularis differs from 

 C. holothuricC, in H. tubulosa, where even sporulating 

 cysts — not yet ripe — occur free. The fact that out of a great 

 many cysts none were loose, although several had formed 

 spores, and also (he delicate nature of the cyst-envelope (see 

 below, p. 36), points to the conclusion that in this species the 

 cyst ruptures in situ, liberating the spores into the coilome ; 



' Tills condition affords an interesting parallel to the layer of yellow cells 

 around tlie intestine of Lunibricus, and it is not improbable that this great 

 development of ccclomic cpitUelium serves a similar purpose in the two cases. 



