132 CLASS TV. 



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 Echinodertns 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 x . 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 2 . 



As little is known hitherto of the course which the water takes 

 in most Holothurice 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-fishes 3 . In other Holothurice, as in those which TIEDEMANN 

 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.] 



2 VALENTIN, op. cit. p. 83. 



3 QUATREFAGES, Op. Clt. p. 65. PI. 5, fig. 7, /. 



