146 ON THE CIRCULATION OF THE BLOOD, 



In fishes the heart is single, or consists only of two compartments instead 

 of four, and hence the circulation is single also. The gills in this class an- 

 swer the intention of lungs, and the blood is sent to them for this purpose 

 from the heart, in order to be deprived of its excess of carbon, and supplied 

 with its deficiency of oxygen. It is not returned to the heart, as in the case 

 of the superior animals, but is immedatiely distributed over the body by an 

 aorta or large artery issuing from the organ of the gills. The oxygen in 

 these animals is separated from the water instead of from the air; and for this 

 purpose the water usually passes through the mouth before it reaches the 

 gills : yet in the ray-tribe there is a conducting aperture on each side of the 

 head, through which the water travels instead of through the mouth. In the 

 lamprey it is received by seven apertures opening on each side of the head 

 into bags, which perform the office of gills, and passes out by the same orifices, 

 and not, as has been supposed, by a different opening said to constitute its 

 nostril. 



In the common leech there are sixteen of these orifices on each side of the 

 belly, which answer the same purpose. In the sea-mouse (aphrodita aculeata) 

 " the water passes through the lateral openings between the feet into the 

 cavity under the muscles of the back."* 



The siren possesses a singular construction, and exhibits both gills and 

 lungs ;f thus uniting the class of fishes with that of arnphibials. Linnaus 

 did not know how to arrange this curious animal, and shortly before his 

 death formed a new order of amphibials, which he called MEANTES, for the 

 purpose of receiving it. It ranks usually in the class of fishes. 



The only air-vessels of the winged insects have a resmblance to the aper- 

 tures of the lamprey, and are called stigmata. In most instances these are 

 placed on each. side of the body ; and each is regarded as a distinct trachea, 

 conducting the air, as M. Cuvier elegantly expresses it, in search of the blood, 

 as the blood has no means of travelling in search of the air.J They are of 

 various shapes and number, and are sometimes round, sometimes oval, but 

 more generally elongated like a button-hole. In the grasshopper they are 

 twenty-four, disposed in four distinct rows. 



The membranous tube that runs along the back of insects is called by 

 Cuvier the dorsal vessel. It discovers an alternate dilatation and contraction : 

 and is supposed by many naturalists to be a heart, or to answer the purpose 

 of a heart. Cuvier regards it as a mere vestige of a heart, without contrac- 

 tions from its own exertion, and without ramifications of any kind : the con- 

 tractions being chiefly produced by the action of the muscles running along 

 the back and sides, as also by the nerves and trachea, or stigmata. Scorpions 

 and spiders have a proper heart ; and as the term insects is now confined by 

 M. Cuvier and M. Marcel de Serres to those that have only this dorsal vessel, 

 or imperfect heart, the two former genera are struck out of the list of insects 

 as given by Linnaeus. fy 



This organ differs very considerably in its structure and degree of simpli- 

 city in moluscous animals. The heart of the teredo has two auricles and 

 two ventricles ; that of the oyster one auricle and one ventricle. In the 

 muscle the heart is not, strictly speakmg, divided into an auricle and ventri- 

 cle, but rather consists of an oval bag, through the middle of which the lower 

 portion of the intestine passes. Two veins from the gills open into the heart, 

 one on each side, which may be considered as the auricles. 



In several of the crustaceous insects of Linnaeus, as, for example, the mo- 

 noeulus and craw-fish, the stigmata converge into a cluster, so as to form 

 gills ; which in some species are found seated in the claws, and in other spe- 

 cies under the tail. These have for the most part a small single heart, and 



* Sir E. Home, Phil. Trans. 1815, p. 260. 



T Homo's Life of Hunter, prefixed to Hunter's Treatise on the Blood, Inflammation, &c. p. xli. 



t En un mot. le sang ne pouvant allerchercher 1'air, c'est Pair qui va chercher le sang. , Legons d'Anat. 

 Comp. i. 23, Sect. 2, Art. 5. 



See M. Marcel de Serres' Statement, Tilloch's Journal, vol. xliv. p. 148 ; and especially Thomson'* 

 Annals of Phil. No xxiii. p. 347, 348. 350. 354. 



