Heffpiration in Invertebrate Animals. 133 



centre, is actively ojierative iu circulating the contents. In the 

 instance of the articulate animal the mechanical conditions are 

 diflferent : the dorsal vessel alone is the active instrument of 

 circulation. 



The circumferential segments of the system cannot contract 

 upon the contained fluid; the central organ therefore in this 

 class is invested with additional strength ; it is adapted for more 

 powerful work. In its peripheric parts the circulatory appai-atus 

 of the articulate animal may be described as inferior to the 

 analogous divisions of that of the Annelid. The conditions 

 are reversed with reference to the centres. This character is 

 not restricted to the aii'-breathing or tracheary Ai-ticulata ; it 

 prevails amongst the Crustacea. From Hunter to Newport this 

 question has formed an arena of microscopic controversy : — are 

 the vessels provided \\ith separate and independent coats, or does 

 the blood only traverse fortuitous channels in the "cellular 

 membrane" of the solids ? It is not devoid of interest, in the 

 study of the material conditions of solid nutrition : it is not 

 difficult of solution : it will be afterwards answered iu detail. 

 No channel through which chylaqueous fluid circulates is con- 

 tractile. In no invertebrate animal, from the highest Annelid 

 to the Zoophyte, does this anatomical character know an excep- 

 tion. It is this character which is extended to the anatomical 

 disposition of the blood-tubes in the Articulata ; it will be sub- 

 sequently shown to belong equally to the circulatory system of 

 the MoUusca. Although the fluids of the Articulate and !Mol- 

 luscan organisms represent true blood, the conduits through 

 which it moves are not detached and independent structui"es. 

 Although more complex and more raised in vital standard than 

 the chylaqueous fluid of the Annelid and the Echinoderm, the 

 blood of the Articulata and Mollusca is less complex and less 

 raised in the scale of composition than that of vertebrated 

 animals. It is thus easy to demonstrate that there obtains a 

 direct and constant relation between the vital standard and 

 chemical composition of the living fluids, and the anatomical 

 characters of the tubular apparatus in which they perform their 

 circulatory orbits. These facts have especial reference to the 

 theory of respiration afterwards to be propounded. 



The muscular system of the Articulata, as compared with that 

 of the Annulose classes, manifests features of great superiority. 

 The muscle-cell is more densely charged with fibrinous con- 

 tents : the property of contractility is far higher in degree. The 

 ceaseless activity of these animals flows from their remarkable 

 muscularity. It is mechanically obvious that such a powerful 

 muscular system as that of the Articulata presupposes an appa- 

 ratus of fixed solids on which to act, and through which to pro- 



