ECHINODERMATA. 



Fig. 22. 



Atlerias avrantiaca opened from above. 



A, ray with thft coeca a, g, in their place. B, cceca removed ; vesicles of feet d, seen. C. vesicles of 

 feet removed to shew the calcareous segments of the ray. D, skin forming roof of the body 

 and rays A, B, C, raised ; vessels seen on its inner surface with collapsed stomach,/, &c. 



inferior circular vessel. The descending canal 

 is dilated in the middle ; its comparatively 

 thick brown coloured parietes are smooth 

 externally, but reticulated on the inside and 

 composed of interlaced fibres, which Tiede- 

 mann found to possess muscular irritability. 

 He accordingly considers this canal as the 

 heart. The inferior circular vessel (which 

 must not be confounded with the circular 

 canal connected with the feet) surrounds the 

 mouth on the outside or inferior surface ; it 

 sends out five branches which pass into the 

 interior of the body, and are distributed to the 

 stomach, caeca and ovaries. Tiedemann re- 

 gards these branches with the circular vessel 

 from which they proceed as arteries, and he 

 thinks it probable that their minute ramifica- 

 tions open into the radicles of the veins, 



though from their delicacy he has not been 

 able to ascertain the fact by injection. 



Tiedemann's view of the function of the 

 respective vessels is derived solely from a con- 

 sideration of their anatomical disposition, and 

 while in the same way it may be inferred that 

 the blood circulates in a direction conformable 

 with this view, it must nevertheless be kept in 

 mind that no direct physiological proof of 

 such a course of the blood has been yet ob- 

 tained. Besides the vessels described, Tiede- 

 mann found yet another circular vessel sur- 

 rounding the mouth on the under surface and 

 placed more superficially than the last men- 

 tioned ; it is of an orange colour and sends a 

 branch along each of the rays, in the groove 

 which is on the middle of their inferior sur- 

 face. He could trace no connection between 



