384 REPTIL1A. 



frequent than those of the Tetracoilia, whether under water or in the 

 common air, they can of course dive much longer : but an animal to 

 he truly amphibious must have its respiratory apparatus compounded of 

 the pulmonary and branchial organs, which is the case with this tribe, 

 for these only can be said when in the air to be truly terrestrial, and 

 when in the water truly aquatic. 



In the description of any very uncommon animal, in which the parts 

 upon which life immediately depends differ very much from those com- 

 monly known, and bear a strong analogy to others which are but very 

 indifferently understood, it becomes necessary to give some idea of the 

 preceding and succeeding links in the general chain of organization, 

 before the connexion of that in question can be distinctly understood. 

 As the animals now before us, therefore, differ remarkably in the circu- 

 lation of the blood and organs of respiration from all others yet known, 

 our first business is to consider the different forms under which those 

 two essential principles of animal life appear. 



The heart is acknowledged to be the grand organ of the motion of 

 the blood in those animals which have it. In some animals it acts a 

 double part, serving also for digestion 1 , but these have no connexion 

 with the present subject. Where its use is confined entirely to the 

 circulation, its structure is different, according to the different motions 

 the blood is destined to receive ; for in some animals the heart has only 

 one, in others two, in others three, and in the most complete of all four 

 cavities ; and this difference of structure forms so many grand divisions 

 of the animal kingdom, which I must be permitted to call by the names 

 of Monocoilia, Dicoilia, Tricoilia, and Tetracoilia 2 . 



The last order comprehends the animals whose hearts have only one 

 cavity, as is the case in the insect, therefore they may be called Mono- 

 coilia ; but as these have no connexion with the present subject, it is 

 sufficient just to mention them. 



The animals of this class have but a single circulation, and of course 

 are furnished only with two cavities to the heart, viz. an auricle and 

 ventricle, similar to and answering the same purposes with the 

 right half of the heart in the most perfect animals. To this order 

 belong all the fish with gills ; therefore these may be called Dicoilia. 



1 [Hunter here alludes to the central cavity of the body of the Medusa. See his 

 injected preparations of the Rhizostoma, Nos. 847, 848, 982, 983.] 



2 [These should be written Monocoela, Dicoela, &c. ; but they are spelt as above in 

 in the ' Treatise on the Blood and Inflammation,' 4to, 1793, p. 135, where these 

 names for the Divisions of the Animal Kingdom were first proposed. Like all 

 classifications founded on the variations of a single organ, the cardiac arrangement 

 is too artificial for general application. For Cuvier's Radiated Division of the 

 Invertebrata, Hunter proposed the Term Acardia.] 



