Respiration in Invertebrate Animals. 249 



chylaqueous fluid to the aerating medium. Arising out of the 

 roof of the stomach, as already stated, at the side of the oral orifice, 

 they can be injected only with the contents of the stomach, and 

 that periodically by muscular force. Such a mechanism, for 

 organs which are indubitably respiratory, is in the highest degree 

 improbable. The tentacles of the distended polype are filled 

 undoubtedly by a fluid. In this fluid no corpuscles have yet 

 been detected. Those observed so readily in the stem cannot be 

 traced upwards beyond the base of the stomach : Prof. AUman 

 denies even in the latter situation the existence of cilia. The 

 globules move, according to this observer, as "the effect of the 

 active processes, going on in the secreting cells of the endoderm, 

 — processes which can scarcely be imagined to take place without 

 causing local alterations in the chemical constitution of the sur- 

 rounding fluid and consequent disturbance of its stability/' How- 

 ever these questions may eventually be determined, it is certain 

 that there exists in all zoophytes but one fluid system. 



This fluid is compounded of the surrounding medium, whether 

 it be sea water or fresh, and the organic products of digestion. 

 By this quasi-inorganic fluid the nutritive functions of the organ- 

 ism are performed. In the tentacles it undergoes aeration ; in the 

 actiniform orders it may be collected in large quantities : it con- 

 tains corpuscles characteristic of species*. It affords distinct evi- 

 dence of the presence of albumen ; it is destitute of fibrine ; it 

 is the lowest example under which a living nutritive fluid occurs 

 in the animal kingdom, and yet the cells of the solids of zoophytes 

 are eminently iiTitable and contractile. An inverse proportion 

 obtains generally in this respect in invertebrate animals. The 

 simpler the fluids, the more irritable and contractile the solids, 

 the cells of the latter being larger than the corresponding parts 

 of verteb rated animals. 



Bryozoa (fig. 4). — The marine and freshwater polyzoa are mol- 

 luscan in the character of their ahmentary system, zooph}i;ic in 

 that of the fluids. Their position in the scale must be allotted 

 according to the relative importance of these two systems : judged 

 by the fluids, they claim to rank at the summit of the zoophytic 

 series ; by the alimentary organs, they would constitute the first 

 link in the molluscan chain. The real signification of the fluids 

 in the Polyzoa has never been understood. A perigastric cavity 

 {a, b) is clearly described ; the fluid within this cavity and its float- 

 ing corpuscles have been repeatedly observed, but the physio- 

 logical value of these parts has never been explained f- In these 



* See the author's papers on the Blood, wliich are now in course of 

 publication in the British and Foreign Med. Ch. Rev. 



t In justification of the statements made in the text, the author would 

 refer to the admirable report, on the Polyzoa, by Prof. Allman, in the Trans. 



