402 BIOLOGY. 



2342. By degrees the cloaca is retracted towards the 

 ureters, and then originates a cloaca, which is both ure- 

 thra and urinary bladder, as in Birds. 



2343. In the higher animals, where a perfect ourocyst 

 or urinary bladder has been evolved, the urethra opens 

 into the anterior wall of the vagina, in front of which the 

 urinary bladder then lies, in a similar manner to the tra- 

 chea in front of the pharynx. 



2344. In many Fishes the urinary bladder is absent, 

 their pulmonic sac being also but feebly developed and 

 only remaining as an asymmetrical swimming-bladder 

 the ureters too open directly into the cloaca, just as the 

 swimming-bladder opens into the oesophagus. The pha- 

 rynx of Fishes, as being surrounded by the branchial 

 arches, is at once pharynx and larynx, like as in many 

 animals cloaca and urinary bladder are of one and the 

 same kind. 



2345. In many Reptiles (such as the Tortoises and 

 Frogs) the urinary bladder gives off two cseca or blind 

 sacs, as is the case in the larynx of many apes. 



2346. In the Bird the two cul-de-sacs of the urinary 

 bladder are much more developed and have assumed the 

 form of two csecal intestines, so that they have been 

 actually viewed as such, and their number two been 

 assigned as characteristic of the bird, the other animals 

 meanwhile having only one. The caeca intestinalia of 

 Birds are the lateral and upper extremities of the urinary 

 bladder. The true caecum of the bird is the vitelline 

 canal or duct, just as in the fishes and all higher or- 

 ganized animals ; this being distinctly retained in the 

 aquatic birds. 



2347. In the Bird the rectum properly opens into the 

 urinary bladder between the two blind sacs or caeca, and 

 that indeed with a regular vulva, which is a sphincter 

 muscle. 



2348. The cloaca of the Bird is an urinary bladder, 

 into which the anus opens. 



2349. The orifice of the cloaca is properly that of the 

 urethra ; ova and faBces are moistened with urine. In a 

 bird both these egesta are combined. 



