12 CLASS IV. 
object of respiration, may be effected in any part of the animal 
organism, where the finest branches of the blood-vessels (the 
Capillaries) are bathed in the medium in which the animal lives. 
Hence it is easy to conceive that this function is not always neces- 
sarily connected with determinate parts. 
In almost all Echinoderms the sea-water penetrates into the 
cavity of the body, and bathes as well the internal surface of the 
integument as the outer surface of the intestines. Where no 
especial respiratory organ exists, the function of such an organ, the 
change of the blood, may be effected in the fine vessels which run 
on the surface of the intestinal tube. 
In the Star-fishes the sea-water penetrates to the cavity of the 
body by means of fine tubules in the integument, which are found 
in great numbers on the dorsal surface’. In Ophiura there are on 
the abdominal surface in each of the five fields between two rays, 
two or four fissures leading into the cavity of the body. 
In the Echini it is not known with certainty in what way the 
water penetrates the cavity of the body. ‘The ten branched organs 
round the mouth, which TIEDEMANN considers to be tubules to 
convey the water in and out, have, according to VALENTIN, no 
external apertures *. 
As little is known hitherto of the course which the water takes 
in most Holothurie to reach their cavity. In those which have no 
special respiratory organ, the genus Synapta has between the ten- 
tacles that surround the mouth four or five small papilliform 
eminences, having an opening at the apex and conducting to as 
many tubules that open between the muscles of the mouth. The 
openings are beset with cilia, like the tubules of the integument in 
Star-fishes3. In other Holothurie, as in those which T1mDEMANN 
investigated, there are special respiratory organs. From the Cloaca 
in which the intestinal canal terminates, there proceeds upwards 
a short tube, that soon divides into two very long principal branches 
which run as far as the anterior part of the intestinal canal. From 
these smaller tubes arise which subdivide into twigs which termi- 
1 [This is TIEDEMANN’S opinion with respect to star-fishes, but the observations of 
SHARPEY, EHRENBERG and MUELLER, are opposed to it; they saw the streams of 
water from within turn back when they reached the extremity of the tubules. ] 
4 ‘VALENTIN, op. cit. p. 83. 
3 QUATREFAGES, op. cit. p. 65. Pl. 5, fig. 7, f. 
ER. ans 
