56 COMPARATIVE ANATOMY. 



RADIATA. Ccelenterata. With some Polypi closed blood- 

 vessels exist upon both sides of the body and stomach : they 

 have proper walls, and circulate a fluid containing a great 

 number of white globules. (Siebold.) Acalephse. Without 

 proper sanguineous system. The so-called vessels would 

 appear to be ramified and anastomosing gastric tubules. An 

 * aquiferous system' is announced in the form of ciliated 

 canals traversing the body and receiving water through 

 stomach, or directly from without, to be ejected through 

 openings upon the extremity of the body and on the margin 

 of the disk (Ibid.). The Coelenterata are without a dis- 

 tinctive respiratory apparatus. 



Echinodermata. Blood-vessel system complete. Its gen- 

 eral expression is a circular vessel surrounding the mouth 

 and anus, communicating with rudimentary hearts and pos- 

 sessed of diverging arterial and venous branches. Heart 

 absent in Holothuria. In Asteracanthion (star-fish) and Ophiura, 

 water passes freely through openings in the skeleton, the 

 currents among the viscera being determined by cilia. This, 

 the aquiferous system, performs in part the function of res- 

 piration. With others, internal branchiae extend upward 

 among viscera to assume an arborescent form (respiratory 

 tree), as in Holothuria (sea-cucumber): a smaller and more 

 tentacular form is seen in Echiurus. 



MOLLUSCA. Blood-vessel system incomplete; heart, with 

 the exception of Cephalopoda, systemic, i. e. one that receives 

 the aerated blood from gills to distribute it through the body. 

 Currents of water over branchial surfaces induced by ciliary 

 action, except in Cephalopoda. 



Polyzoa. Respiration effected through tentacles surround- 

 ing mouth. 



about half as broad as body. Upon contraction occurring, the radiated 

 canals appear as fine streaks, which gradually broaden as the main portion 

 lessens, until the former is emptied, and nearly invisible. As the main 

 vesicle begins to expand, the canals, in nearly regular sequence, suddenly 

 open and their contents pour into the central chamber. They then gradu- 

 ally disappear while the vesicle again contracts. (Clark.) 



