ECHINODERMS. 131 



lymphatics, run upon the mesentery to form a stem, having a 

 curved course, from which other vessels arise to run to the respira- 

 tory organ and so may be named pulmonary arteries. With these 

 pulmonary arteries the pulmonary veins are in connexion, from 

 whose union a longitudinal stem arises from which branches proceed 

 to the arterial vessel with which we began our description l . 



Besides the blood-vessels already described there are other 

 vessels which in Echinoderms provided with suckers or feet are in 

 connexion with these organs of motion. The integument of the 

 body is perforated by numerous pores arranged regularly in rows ; 

 in the sea-urchins the rows have been called, on account of their 

 regularity, Ambulacra, from a comparison with orderly rows of 

 trees and garden- walks. Through these pores membraneous cylin- 

 drical feelers (the feet) pass out, each terminating in a minute 

 suctorial disc. According to the investigations of VALENTIN these 

 feelers are in Echini perforated at their extremity by a fine aperture. 

 Within the integument there are vesicles in connexion with them. 

 The feelers, hollow within, are filled with a fluid, usually sea- 

 water, which the animal can press at will from the vesicles, or, by 

 contraction of the former, can cause to flow back. In this way the 

 animals move their body, the numerous feet contracting and elon- 

 gating, and adhering by means of the suckers. There are vessels 

 corresponding to the rows of feet or feelers, from which lateral 

 branches proceed to the vesicles of the feelers. The ordinary 

 number of these longitudinal vessels of the integument is five ; in 

 the star-fishes their number corresponds with the number of the 

 rays of the body. These lymphatics fall into an annular vessel 

 surrounding the mouth. In Holothuria the appendages of the 

 feelers which surround the mouth proceed from this annular vessel : 

 and from it there arise also five other vessels that descend along the 

 commencement of the intestinal tube, where they terminate in 

 another annular vessel from which one or two oblong csecal vesicles 

 depend (Ampulla Poliana), that are in like manner filled with 

 watery fluid 2 . 



The change of the blood from venous to arterial, the proper 



1 See TIEDEMANN, Anat. der Eohren-HolotJiurie, s. 1518, Tab. m. ; comp. also 

 CUVIER, Regne Anim., edit, ill., Zoophytes, PI. 18. 



2 See the figures in TIEDEMANN, Tab. in. fig. 4, 6. 



92 



