176 ECHINODERMATA, 



abdomen, to the walls of which it is attached by delicate fleshy 

 bands : this cloacal cavity communicates with the exterior 'of the 

 body by a wide orifice twice as large as the aperture of the mouth, 

 through which, in the figure, a bristle (f) has been passed ; it is 

 by this hole that the water required for the purpose of respiration 

 is taken in, and it is then forced by the muscular walls of the cloaca 

 itself through the whole system of respiratory canals by which its 

 distribution is effected. The organs of respiration commence at 

 the upper part of the cloaca, near the termination of the intes- 

 tine, by a large opening leading to a wide membranous tube, which 

 immediately divides into two vessels (g, g,) forming the main 

 trunks of the beautiful arborescent branchiae, which extend to the 

 opposite extremity of the body, giving off in their course numerous 

 lateral branches that divide and subdivide, so as to form what has 

 been not inaptly termed the " respiratory tree," until they ulti- 

 mately terminate in minute vesicular caeca, into which the water 

 derived from the cloaca of course penetrates. One division of this 

 elegant apparatus is maintained in close contact with the walls of 

 the body by a series of delicate tendinous bands, while the other 

 becomes applied to the convolutions of the intestines, with which 

 it is likewise united. It is this last-mentioned division which 

 would appear to be specially provided for the oxygenization of 

 the nutritive fluids taken up by the intestinal veins. 



(217.) The circulation of the blood in the Holothuria, as in 

 the Echinus, is still but imperfectly understood, and considerable 

 difference of opinion upon this subject will be found in the 

 writings of anatomists. According to Tiedemann,* innumerable 

 small veins collect the blood and nutritive products of diges- 

 tion from the intestine, and convey them into a large central 

 vessel, (Jig. 74, i, i,) from whence the circulating fluid passes 

 by other trunks (/, /,) to the respiratory tree ; hence it is re- 

 turned by vessels (partly represented at m) to the intestinal 

 artery (k) 9 by which it is again distributed over the intestinal 

 parietes. 



Delle Chiaje gives a different account of the arrangement of 

 the vascular system in these creatures, which he seems to have 

 investigated with his usual untiring perseverance. According to 

 the last-mentioned anatomist, the blood is taken up from the in- 

 testines by a complicated system of veins, the main trunks of which 



* Anat. der Rohren, Holothuriej fol. 1816. 



