An Introduction to Zoology. 33 



with the dorsal, or as it may justly be called the cordal trunk, a more perfect 

 vascular system of circulation ; and as this affords facilities for carrying 

 the blood to particular localities for the purposes of aeration, the all-jjer- 

 vading ramifications of the numerous trachefe are no longer found ; but in 

 their place four pulmonary cavities communicating with the external air on 

 either side, and covered with ramifications of blood vessels, which issue from 

 the cordal trunk, and thus expose the blood within them to the action of 

 the atmosphere. Such cavities are in fact lungs ; and thus we find in the 

 arachnida the simplest form of lungs for direct aerial respiration. 



Nearly allied to the arachnida are the Crustacea, in which the system is 

 still further advanced. 



In the Crustacea we have, not only the arterial system spoken of, but the 

 two great venous trunks ; these venae cavae, ranging on either side, send out 

 subsidiary branches which issue in innumerable ramifications, spread over 

 a fringe-like or filamentous structure, one of each of the fringes being at- 

 tached to the haunches of each of the thoracic feet. This constitutes a gill, 

 the common organ of aquatic respiration, by which the water charged 

 with air, is brought into contact with a thousand minute vessels, whose 

 expanded structure thus exposes a very considerable aggregate of surface 

 to its influence, and thus accomplishes the ends of aquatic respiration. 

 Such aquatic organs, or gills, only differ from aerial lungs, in that the 

 membranes upon which the vascular ramifications are conducted, are in the 

 latter case wrought into a congeries of minute cellules, into the interior of 

 which the ultimate ramifications of the trachea open and admit the air. 

 As to the precise mode of communication or contact between the air 

 and the circulating fluid, we are not called upon now to deliver an opinion. 

 The matter is a curious one, and will probably form the subject of a sepa- 

 rate paper. 



In the Crustacea, the branches sent out from the venae cavse to the gills 

 or branchire, dilate at their origin into muscular contractile sinuses, ac- 

 celerating the current of the blood, and therefore to be considered as so 

 many branchial hearts, the vessels springing from them being in fact the 

 pulmonary arteries ; and as these arteries diffuse the blood over the 

 laminated structure of tiie gills, so the corresponding veins convey the 

 blood back again after it has undergone the changes attendant on aeration, 

 and deposit it finally in a concentrated form in a large oval cordal vessel. 

 In the lobster this vessel begins to assume the character of a true heart, 

 dispersing its blood by means of an aorta and arteries, in an aerated con- 

 dition, through the system, and affording a very perfect type of ordinary 

 branchial respiration. 



We may here, in proceeding, observe how impossible it is to arrange 

 the animal kingdom in any thing like a linear scries with reference to the 

 gradually advancing perfection of development of these functional organs ; 

 and how loose is that theory which would regard the species as having 



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