164 



in different species, entitling these bodies to great consideration in 

 the establishment of species. 



The paper then proceeds to demonstrate the proposition, that in 

 those classes, as in the Echinodermata, Entozoa and Annelida, in 

 which, in the adult animal, these two orders of fluids coexist, though 

 distinct, in the same individual, there prevails between them, as 

 respects their magnitude or development, an inverse proportion ; that 

 while, as instanced in the Echinoderms, the chylo-aqueous fluid 

 filling the ciliated space between the stomach and integument is 

 considerable in volume, the blood-proper and its system are little 

 evolved ; that while, as in the Entozoa, the chylo-aqueous fluid is 

 still the most important fluid element in the organism, the blood 

 system is proportionally rudimentary ; that in the Annelida, espe- 

 cially the higher species of that class, the chylo-aqueous fluid almost 

 disappears, while the system of the true blood acquires, illustrating 

 the law of inverse proportion, a correspondingly-augmented develop- 

 ment. The author then states, that the system of the ch3do- aqueous 

 fluid does not exist in the adult, but only in the larva state of the 

 higher members of the articulated series, such as the Myriapoda, 

 Insecta and Crustacea. 



In Myriapods and Insects, he has observed that the peritoneal 

 space is occupied by a fluid which does not communicate with, and 

 is distinct in composition from, the contents of the true blood-vessels. 



This peritoneal fluid, however, in these classes disappears at a 

 subsequent stage of growth. Thus the author thinks that a con- 

 tinuous chain, through, the medium of the fluids, is established be- 

 tween the Echinoderms at one extreme and the Crustacea at the 

 other. These classes he proposes to connect together under the 

 designation of the double fiuid series, corresponding to the radiate 

 and articulate series of systematic zoologists. 



Returning to the standard of the Echinoderms, where the system 

 of the blood-proper first appears in the zoological scale, he shows 

 that at this point the Molluscan chain diverges from the radiate and 

 articulate chain, and may be indicated, in contradistinction from 

 the latter, as the single-fluid series. The author's observations lead 

 him to believe, with Professor Milne-Edwards, that in all Molluscs, 

 from the Tunicata to the Cephalopods, the chamber of the perito- 

 neal is continuous with the channels of the circulation, and that 

 consequently the fluids observed in these parts are one and the same 

 fluid, establishing the singleness of the fluid system of the body ; and 

 this conclusion is corroborated by additional evidence drawn from 

 microscopic examinations. 



He then recapitulates the results of his researches, and maintains 

 that the base of the invertebrated kingdom of animals is formed of 

 all those inferior series which rank below the Echinoderms ; and 

 that this series is distinguished from the Molluscan, in which also 

 the fluid system is single, by the important circumstance that in the 

 former, unlike the Mollusca, the digestive and circulatory system 

 are identified, or confounded into a single system ; that at the Echino- 



