ORGANS OF GENERATION IN THE INVERTEBRATA. 97 



mal. The peculiar grunting voice of the pig is produced by large 

 lateral cavities communicating with the small ventricles of the 

 larynx, and the neighing of the horse results from the vibrations of 

 membranous folds connected with the chordae volcales. 



The trachea varies considerably in the length, breadth, and num- 

 ber of its rings; thus, in the seal, the porpoise, the cheiroptera, and 

 several rodentia, its rings form complete circles as in birds ; their 

 number varies from 14, presented by the mouse, to 78, as seen in the 

 seal. In the sloth the trachea descends considerably in the chest, 

 and again ascends to divide into the bronchi. The lungs present 

 very few varietes in the class mammalia. In the cetacea they 

 are remarkable for their elongated, flattened form, and for the free 

 communication of their cells with each other. 



RECAPITULATION. 



1. The respiratory apparatus is very extensive and greatly varied 

 in the invertebrata, in the lowest orders of which it is usually con- 

 fined to the surface of the body. 



2. Respiration is performed by gills in fishes, and in the caduci- 

 branchiate amphibia, during the tadpole state ; in the siren and 

 proteus it is in all probability performed all through life both by 

 gills and lungs. 



3. In birds this system is extended into the bones and into the 

 large cells of the thorax and abdomen. 



4. The organs of respiration throughout the class mammalia, 

 are very similar to those of man. 



CHAPTER XVII. 



ORGANS OF GENERATION IN THE INVERTEBRATA. 



Notwithstanding the varied modifications observed in the organs 

 of generation throughout the animal kingdom, the reproductive 

 function will be found to conform to a few leading types, as the 

 fissiparous, sfemmiparons, oviparous, viviparous, and ovo-vivipa- 

 rous. Fissiparous generation consists in the division of an animal 

 into two or more, similar in every respect to the original being. In 

 this form of generation, which is met with in some of the infusoria, 

 cestoidea, and annelida. the line of separation takes different direc- 

 tions, being transverse in the paramoecuim, and vertical in the vor- 

 ticellae. In gemmiparous generation the young appear as sprouts 

 from the body of the parent: it is more extended than the last, 

 being met with in the polypine and coralline animals, in sponges, 

 cystiform entozoa, and in some acalephae. These two forms of 

 9 — e evers 7 



