250 Dr. T. Williams on the Mechanism of Aquatic 



animals there existsneither a heart nor a blood-proper system. The 

 fluids constitute an unmixed example of the chylaqueous system. 

 They oscillate under muscular agency in the great visceral cavity 

 {a,b); under the same force the fluid penetrates the tentacles which 

 it traverses by a flux and reflux motion. These organs are plain, 

 tubular appendages ; they are continuations of the visceral cavity ; 

 — characters which are emphatically zoophytic. The tentacles of 

 the Polyzoa difier from those of asteroid polypes in the presence 

 of vibratile cilia. They are limited to the external surface, and 

 arranged in a single row on either side : the interior of these 

 branchial tubuli is not ciliated. Thus then is defined the whole 

 apparatus of the chylaqueous system in this family. Hence- 

 forth the Polyzoa cannot be severed from the zoophytes. 



Acalepha (figs. 5, 6, 7)- — The apparatus for breathing is, in 

 this class, of simple construction : it consists of a system of csecal 

 canals in direct connection with the stomach. Four types occur 

 — the Pulmograde, the Ciliograde, the Cirrhigrade, and the Phy- 

 sograde. In the first examples [Aurelia, Pelagia, Chrysaora, Rhi- 

 zostoma, Cassiopea and Cyaiued), the stomach is a central lobu- 

 lated chamber, furnished with one external orifice, the mouth, 

 and opening laterally into canals which reticulate at the margin 

 of the disc : they end csecally. In Cyaneea aurita, they are pro- 

 longed into the fringed appendages which depend from the cir- 



of the Brit. Assoc, 1850, in which the following statements occur. " The 

 perigastric space and interior of the tentacula and locophore all freely 

 communicate with one another, and are filled with a clear fluid, in which 



float numerous irregular particles of very irregular form and size That 



the fluid thus contained in the perigastric space, and thence admitted into 

 the tentacles, consists really of water which had obtained entrance from 

 without, there can, I think, be little doubt ; and yet I have in vain sought 

 for any opening through which the external fluid can obtain admittance 



into the interior The fluid which circulates in the perigastric space is 



not perfectly homogeneous, and numerous corpuscles of vai-ious and irregidar 

 shape may be observed to float through it and be carried about by its 

 current. Some of these coi-puscles are perhaps spermatozoa ; others are of 

 no definite shape, and look like minute portions of the tissues separated by 

 laceration. May they not be some of the products of digestion, which have 

 transuded through the walls of the alimentary canal, being thus conveyed 

 into the only representative of a true circulation, with which these animals 

 present us ? " From the preceding passages it is undeniable that this ex- 

 cellent naturahst has not clearly seized the significance of that which he 

 has described so graphically. He admits that the fluid of the perigastric 

 cavity is the only fluid system discoverable in the organism of the polyzoon. 

 He disputes the organic character of the fluid, while he hints at its nutritive 

 properties. It is in truth a true and perfect chylaqueous system, and as 

 adequate as blood-proper to the wants of the living organism. In the 

 Polyzoa, there is discoverable no trace of a blood-proper system. They 

 therefore fail in one of the most essential characters of the molluscan or- 

 ganism — the existence of a heart and an associated circulatory system. 



