ALIMENTARY CANAL OF VERTEBEATA. 



539 



auditory meatus." The external auditory meatus in the Mammalia 

 is different, for its deeper portion is formed 

 by the tympanic bone. The external 

 ear, tlie cartilaginous support of which, is 

 continuous with, a narrow auditory meatus, 

 is attached to this. There is no external 

 ear in the Monotremata. The "extei-nal ear" 

 may be much modified, either in form, or 

 in its relations to the muscular appai^atus, 

 which moves it. In addition to the muscles, 

 which move the whole of the external ear, '^■■|l/j 



and which are sometimes of much power even , 



in Mail, there are others which are placed 

 in the cartilage of the ear itself ; these are 

 partly represented, though of course as rudi- '* 



mentary organs, in the human ear. This \\ \/, 



external ear is still more atrophied in aquatic ; 



Mammalia. Reduced in Otaria, it is alto- 

 gether absent in the rest of the Pinnipedia, 

 as it is also in the Sirenia and Cetacea. 



Alimentary Canal. 

 §404. 



The alimentary or enteric canal of the 

 Vertebrata forms a tube, which runs be- 

 low the axial skeleton, and in which two 

 chief portions can be distinguished mor- 

 phologically, as well as physiologically, 

 at a very early epoch. The most anterior 

 portion is directly connected with the 

 body-wall, and, as it is perforated by 

 branchial slits, it functions as a respira- 

 tory organ, for respiratory apparatuses are 

 developed in the vascular arches between 

 the clefts. This portion does not, therefore, 

 belong exclusively to the digestive organs, 

 although it is used in the ingestion of food. 

 It forms the respiratory cavity, at the 

 end of which the nutrient canal, in the 

 strict sense, commences ; this is separated 

 from the body-wall by the pleuro-peritoneal 

 cavity. The Vertebrata have these 

 two portions of the enteric tube in 

 common with the Tunicata. In the 

 Acrania the respiratory chamber of the 

 enteric tube occupies a very large portion, 

 which, as in the Ascidiee, represents a large 



Fig. 303. Aniphioxiis 

 lanceolatus (x2^). 

 a MouthjSnrrounded by cirri. 

 Z) Anus, c Abdominal pore. 

 S, Branchial sac. e Gastric 

 portion of the enteron, 

 / CEecnm. (j Hind-gut. 

 7i Coelom. i Notochord, be- 

 low which is the aorta, which 

 accompanies it for nearly 

 its whole length. U Aortic 

 arches. I Aortic heart. 

 m, Enlargements of the 

 branchial arteries, n Heart 

 of the vena cava, o Heart 

 of the portal vein (after 

 Quatrefages). 



