362 Mr. R. HilPs Contributions to the 



In all animals whose respiratory organs are so constructed, 

 that the consumption of oxygen, and the consequent evolution 

 of carbonic acid gas is minute in quantity, the production of 

 heat is proportionally small. In the invertebrate classes the 

 respiratory apparatus is feeble in its action, and these animals 

 accordingly generate heat in a minimum degree. In the class 

 of fishes, though the respiratory apparatus is large, and though 

 all the blood of the body circulates through it, yet as only the 

 air contained in the water is brought into contact with the re- 

 spiratory organ, the temperature of the blood is, as a consequence, 

 regulated by that of the fluid in which the fish swims. In the 

 reptile, though there is a true and proper lung, and though air 

 is respired, yet as only one half of the blood of the body 

 circulates through the comparatively small, imperfectly divided, 

 and simply constructed air-bag, which constitutes its respiratory 

 organ, its temperature does not exceed that of the atmosphere. 

 Hence the contrast exhibited between the temperature of cold- 

 blooded and warm-blooded creatures — between the mammiferous 

 quadruped, whose lung, comparatively large, and composed of 

 innumerable minute and closely- set air-vesicles, presents to the 

 atmosphere an immense extent of surface, and the intermediate 

 air-breathing reptile, whose organs, made up of numerous 

 divisions broken into vesicles and cells, bring the circulatory 

 fluid but imperfectly into contact with the air. In the fish, the 

 lowest order of Vertebrata, the respii'atory organs, formed as 

 fringed folds disposed in leaves, and called gills, communicate 

 to the blood increased warmth only by adding the direct action 

 of the sun's rays to the augmented heat of the air in contact 

 with the superficial waters. 



As a resort to the warmth of the surface waters is necessary 

 to mature the foetus in the viviparous Shark, the period of 

 gestation must be increased or diminished just in proportion as 

 facilities are afforded for the access to heat. " Les femelles," 

 says Bosc "mettent bas leurs petits successivement et k des 

 epoques plus ou moins eloignees, selon les especes, et sans doute 

 selon la chaleur de Feau au milieu de laquelle elles vivent." It 

 is necessary to detail the generative organs of these viviparous 

 fishes to understand the exigencies of this oeconomy. 



I shaU avoid a minute account of the general internal organs 

 of Sharks. It is enough to mention that immediately beneath 

 the heart and liver, which are situated near the mouth of the 

 fish, and between the arches that support the respiratory ap- 

 paratus, the testes of the male, attached to the region of the 

 spine, communicate with long and tortuous vasa deferentia, which 

 occupy the whole length of the body to the cloaca. Correspond- 

 ing to the testes of the male are the ovaria of the female, which 



