162 THE ANATOMY Or VEETEBEATED ANIMALS. 



ophthalmic artery, which pierces the sclerotic, and breaks 

 up into another rete inirabile, the choroid gland, befoi'e being 

 finally distributed. 



In the Lamprey, as has been seen, the respiratory organs : 

 are pouches, the anterior and posterior walls of which are 

 raised into vascular folds. The walls of adjacent pouches 

 are distinct and but loosely connected together ; and con- 

 siderable spaces of integument sepai-ate their rounded outer 

 apertures. 



In the ordinary Elasmobranchii, the branchial pouches 

 are more flattened from before backwards, and their outer 

 apertui-es are more slitHke. The integumentary spaces 

 between the slits are correspondingly narrower, and the 

 adjacent walls of successive pouches are more closely ap- 

 l^roximated, so that they are divided only by septa; but 

 the vascular plaits of the surface of the respiratory mucous 

 membrane do not reach the outer edges of these septa. 



In Chimcera, the free edges of the septa are exceedingly 

 naiTOw, and the apices of the branchial processes extend 

 outwards to them. 



In the Sturgeon, the septum is not more than three-fourths 

 as long as the branchial processes, the apices of which are 

 consequently free. 



The process of reduction is earned still further in the 

 Teleostei — the septum not attaining to more than one-third 

 the length of the branchial processes ; and, as in the 

 Ganoids, each process is supported by an osseous or cartila- 

 ginous skeleton. 



The Teleostei have no functional hyoidean, or opercular, 

 gill; and, as a general ride, each of their four branchial 

 arches possesses a double series of branchial processes, 

 making eight in all. Not unf requently ( Coitus, Cyclopterus, 

 Zens, &c.), the number is reduced to seven ; the fourth bran- 

 chial arch having only one series, the anterior. In this 

 case, the gill-cleft, which should lie between this arch and 

 the fifth, is closed. Sometimes there are only six series of 

 branchial processes,, the fourth a^ch being devoid of any 



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