Respiration in Invertebrate Animalsi I^ 



centre_, is actively operative in circulating the contents.' "^^fn-^he 

 instance of the articulate animal the mechanical conditions are 

 different : the dorsal vessel alone is the active instrument pi 

 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 apparatus 

 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 air-breathing or tracheary Articulata ; it 

 prevails amongst the Crustacea. From Hunter to Newport this 

 question has formed an arena of microscopic controversy: — are 

 the vessels provided with 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 in 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 Mollusca. Although the fluids of the Articulate and Mol- 

 luscan organisms represent true blood, the conduits through 

 which it moves are not detached and independent structures.i 

 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 ^ 

 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 tM^ 

 theory of respiration afterwards to be propounded. ^ ^ 



The muscular system of the Articulata, as compared with thajr 

 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- 



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