206 



ECHINODEKMATA. 



Fig. 102. 



in minute vesicular caeca, into which the water derived from the cloaca 

 of course penetrates. One division of this elegant apparatus is main- 

 tained 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, wherewith it is likewise united. Tt is this last-men- 

 tioned division that would appear to be specially provided for the oxy- 

 genization of the nutritive fluids. 



(537.) The circulation of the blood in theHolothuria, 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. Accord- 

 ing to Tiedemann*, innumerable small veins collect the blood and 

 nutritive products of digestion from the intestine and convey them into 

 a large central vessel (fig. 101, i i), from whence the circulating fluid 

 passes by other trunks (I I) to the respiratory tree ; hence it is returned 

 by vessels (partly represented at m) to the intestinal artery (Jc), by 

 which it is again distributed over the intestinal parietes. 



(538.) Delle Chiaje gives a different account of the arrangement of 

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

 gated with his usual untiring 

 perseverance. According to 

 the last-mentioned anatomist, 

 the blood is taken up from the 

 intestines by a complicated 

 system of veins, the main 

 trunks of which are indicated 

 in the annexed diagram (fig. 

 102) by the letters c, e, p p, 

 q q ; these communicate with 

 each other not only by the 

 intervention of numerous 

 anastomosing branches (d d), 

 but likewise by means of de- 

 licate vascular plexuses (a) 

 passing between them. All 

 these veins terminate in two 

 large venous canals (o) that 

 convey the blood and nutri- 

 ment absorbed from the in- 

 testine to a vascular circle 

 (g) placed around the com- 

 mencement of the oesophagus, 

 which corresponds with the 

 circular vessel around the 

 mouth of the Echinus. This circle Delle Chiaje regards as the centre 

 * Anat. der Rohren-Holothurie. FoL 1816. 



Plan of the circulation in Holothurin, according to 

 Delle Chiaje. 



