526 ORGANS OF RESPIRATION. 



body is covered with innumerable minute transparent colour- 

 less fleshy tubes, which, in the living state, are seen to rise 

 and sink incessantly through openings of the skin, conveying 

 water by their ciliated parietes, for respiration, into the inte- 

 rior cavity of the body. Though very minute, these respira- 

 tory tubes resemble in form, the large inferior prehensile feet 

 extended through the ambulacral apertures, being, like 

 them, nearly cylindrical and provided with terminal open- 

 ings. The whole exterior and interior surfaces of the body, 

 and of its internal organs, and of the long ambulacral feet, 

 are likewise provided with vibratile organs and aerated 

 by ciliary currents, like the mucous and serous surfaces of 

 other radiata, and of most aquatic invertebrata. They cover 

 the exterior of the prehensile tubular feet of echinoderma, 

 and even the investing cutaneous membrane of the spines 

 on the exterior of echini. The prehensile ambulacral feet are 

 extended by having fluid injected into them from vesicles at 

 their base, and these communicate by membranous canals 

 with a vascular apparatus disposed around the mouth. The 

 surrounding element enters the peritoneal cavity by nume- 

 rous small membranous tubes, disposed around the lower 

 aperture, and bathes the abdominal viscera, in the echinida, 

 as in the asterida, and the vibratile cilia covering all the in- 

 ternal and external parts of the body, incessantly renew the 

 stratum of water in contact with their surface, so as to aerate 

 every part of the vascular system and the various tissues of 

 the body. 



The whole irritable exterior surface of holothuria is res- 

 piratory, and the ciliated tubular feet which extend in lon- 

 gitudinal rows from its surface, and the ramified, ciliated 

 sheathed tentacula which surround the mouth, as the external 

 parts of other echinoderma and most radiata, as seen in the 

 annexed figure of holothuria spinosa (Fig. 143. A. B. C.) from 

 Port Jackson, of a red colour, and covered with calcareous 

 spines, like an asterias ; but the internal apparatus ap- 

 propriated to this function, and their communication 

 with the surrounding element, are more circumscribed, 

 and they approach nearer to the type of these organs in 

 higher animals. Instead of entering the general cavity 

 of the peritoneum by numerous minute orifices, as in 

 asterida and echinida, the water is inspired by holothuria 

 solely through the cloacal aperture (143. B. C. e. <?.), 



