108 Dr Allen Thomson on the Vascular System 



are attached three small cartilaginous arches analogous to 

 branchial plates or hoops. Of the spaces between these arches, 

 in which, had there been gills, one would naturally have 

 expected to find branchial apertures, the two anterior are 

 closed up by the membrane lining the pharynx and by integu- 

 ments, and the posterior only remains open, forming a perma- 

 nent aperture in the neck in every respect analogous to the 

 branchial apertures of the proteus, siren, and salamanders. 

 Cuvier states, that the examination of the soft parts, as well as 

 of the skeleton, leads him to believe, that at some previous pe- 

 riod of its existence, this animal possesses external gills, similar 

 to those of the larva of the aquatic salamander. The observation 

 of the development of the amphiumas in the foetal state alone, 

 however, will probably enable us to solve this question, as very 

 small specimens (three inches long) have been seen in which no 

 vestige of gills was to be found. 



The lungs of the proteus consist of two oval membranous 

 sacs, situated in the posterior part of the abdomen, which are 

 generally only about a twelfth of the length of the body, and 

 are each connected with the pharynx by a long and narrow tube. 

 These sacs are quite smooth in their interior, and are not sepa- 

 rated into compartments by membranous septa. 



The lungs of the siren and amphiuma, on the other hand, are 

 proportionally much larger than those of the proteus. Those of 

 the siren, according to John Hunter, consist of " two long bags 

 on each side, which begin just behind the heart, and pass back 

 through the whole length of the abdomen, nearly as far as the 

 anus. They are largest in the middle, and honey-combed on the 

 internal surface through their whole length." According to 

 Cuvier, the lungs of the amphiumas are formed by two long 

 cylindrical and very vascular sacs, slightly dilated at the poste- 

 rior extremity. In neither siren nor amphiumas are there any 

 proper trachea or bronchi. 



The heart of these animals seems to hold an intermediate 

 place between that of fishes and batrachia. It consists of a strong 

 fleshy ventricle, a large membranous auricle situated somewhat 

 anterior to and above the ventricle, and a strong muscular bulb, 

 from which the rising aorta springs. 



In the proteus and siren, so far as has as yet been ascertained, 



