CLASSIFICATION OF ANIMALS. 7 



of circulation, compared with lower or higher conditions of the same 

 organ, could never have taught that fact ; — the knowledge of it 

 necessitates and pre-supposes a knowledge of the relation of the 

 heart to the lungs. 



In [the fish the bipartite heart is so connected with the breathing 

 organs, that it transmits exclusively to them the blood which the 

 auricle receives from the veins of the body : it is an organ for the cir- 

 culation of venous blood ; in other words, a " pulmonic heart." An- 

 other question then arises. Why is the dicoelous heart in one animal 

 systemic, in another animal pulmonic ? This can only be answered 

 by a further insight into the organisation and powers of such animals. 

 With respect to the instances adduced, both species are cold-blooded, 

 and, compared with the warm-blooded classes, both have a low amount 

 of respiration ; but the fish and snail differ widely in the degree in 

 which they exercise or enjoy the respiratory functions. The snail, 

 proverbially sluggish and inactive, has its muscular system reduced 

 almost to a single ventral disc, by the successive contractions of the 

 parts of which it glides slowly along. The chief mass of its body is 

 made up of the organs of the vegetative function. We see here a wide 

 convoluted alimentary canal, an enormous liver, a large ovarium and 

 as large a testis combined with many singular accessory generative 

 organs, in the same common visceral cavity : they make up the great 

 bulk of its body. The tissues of such viscera are endued with little 

 of that action which assists in the acceleration of the currents of blood, 

 through them ; and, therefore, the greater circulation is aided by the con- 

 tractions of a ventricle: whilst as the function of respiration bears ever 

 a direct ratio to the energy and frequency of muscular action, it suffices 

 that the venous blood should flow with an equable and unaccelerated 

 stream over the oxygenating surface, and the energies of the heart 

 are therefore confined to the service of the general circulation. 



In the fish, the proportions of the muscular and visceral parts are 

 reversed : the greater part of the body is composed of the vibrating 

 and contractile fibre, by the action of which the fish is propelled through 

 the liquid medium ; while at the same time the systemic circulation 

 is proportionally aided and accelerated. But this amount and energy 

 of muscular action requires a proportional activity of the respiratory 

 function, and the forces of the heart are, therefore, concentrated upon 

 the gills. 



Thus we perceive that a similar construction of an organ may, through 

 its different relations with other organs, subserve different functions : 

 whilst the conditions of such differences demand for their elucidation, 

 a knowledge of the general organisation and endowments of the entire 

 animal. 



B 4 



