Respiration in Invertebrate Animals. 321 



consist of the abdominal cavity, portioned off in order to aid in 

 the circulation of the fluids. Whatever may be the embryonic 

 significance of these roomy dilatations of the blood-channels in 

 this clasSj it is quite certain that in very few instances are they 

 situated external to and around the viscera (a position invariably 

 occupied by the chylaqueous fluid), and therefore in the space 

 bounded by the peritoneum and immediately underneath the 

 external covering *. By the fact of their situation, these parts 

 are then excluded from all participation in the office of breathing. 

 The preceding facts affect the present inquiry in the following 

 manner : — They prove that the organization of the posterior or 

 abdominal portion of the body in the Conchiferous Gasteropods, 

 that part which is lodged within the shell, is not adapted for the 

 admission of water into the interior of the animal, either for the 

 supply of an aquiferous system or for the replenishing of the 

 nutritive fluids. They prove, independently of other evidence, 

 that the water with which the spire of the shell is filled, and in 

 which floats the corresponding portion of the animal, cannot 

 penetrate in any manner into the body ; that it cannot be viewed 

 as a reservoir whence is drawn the contents of a water-vascular 

 system — if such a system has a real existence in these animals ; 

 that, in fact, it can only act in a mechanical sense by enabling 

 the tenant to vary the specific weight of his house, to move 



internally with a mucous membrane, like that which invests the branchial 

 chamber. These and other facts drawn from the adult anatomy of the 

 MoUusca, prove that, whatever argument may be drawn from embryonic 

 development, that the open spaces, lacunae, &c., which arise in the venous 

 segment of the system, ought not too readily to be explained as merely 

 cut-off portions of the perivisceral or peritoneal cavity. 



If the spacious cephalic and pharyngeal lacunae, first defined by Milne- 

 Edwards, be not, in an embryonic sense, spaces cut off from the general 

 cavity of the body, they must ab origine be parts of the vascular system. 

 For the present this subject must remain an open question ; that is, it 

 cannot at present be confidently stated whether the circulating system of 

 Mollusca form a distinct and independent system in the organism, or 

 whether it be only a modified adaptation of the peritoneal cavity, or 

 whether it be a fusion of both. To solve such doubts by an easy reference 

 to the embryological relations of the parts were unsatisfactory. Two parts 

 may have a common point of departure in the process of development, and 

 yet they may stand very remotely apart in their ultimate structure and 

 purpose. How little explanative it is, for instance, to remark, that the 

 * blood- proper ' system of the Annelids and the tracheal system of Insects, 

 being developments of the teguraentary, epidermal layer, are therefore 

 homologous anatomically, and analogous in office ! This, however, is not 

 the occasion for the full discussion of this subject. 



* It should be stated, however, that in the lowest Mollusca, as exem- 

 plified by Firoldides and Atlanta, the space between the integuments and 

 the viscera is described by some observers as forming a constituent arc of 

 the fluid-system. 



Ann. ^^ Mag, N. Hist. Ser. 2. Vol. xvi. 22 



