A Contribution tu tlic Anatomy of Siren lacertina. (^9| 



This organ, as well as the highly developed nose, is doubtless 

 compensatory for the imperfect vision. 



Conclusions. 



I will add here, for the convenience of the reader, a brief sum- 

 mary of what seem to me the most important points brought out in 

 the preceding paper. 



I. The trigeminus nerve possesses four main branches in- 

 stead of the customary number of three. The occurrence of four is 

 well known to be a characteristic of the fish, but up to the present 

 time has been demonstrated in only one animal above them, Siredon, 

 which in many other ways has been shown to curiously correspond 

 with Siren. Excluding the fish, and these two amphibians, variation 

 from the number three is unknown. 



II. The larynx exhibits, anteriorly, a remarkably typical form. 

 It consists of a tube of circular muscular fibres, interrupted dorsally, 

 ventrally and laterally by four raphes of connective tissue, being thus 

 divided into four quarters. More posteriorly two lateral cartilages, 

 the arytenoids, occupy the place of the lateral raphes, thus modifying 

 the simple condition. After a study of the above relations it seems 

 apparent that the anterior portion , with its simple muscular tube, 

 represents an older phylogenetic form and that the cartilage, ap- 

 pearing as it does in the place of the lateral raphes, is a 

 condition acquired later. 



Granting this as a reasonable hypothesis, the following deductions 

 may be drawn: 



a) That the primitive larynx consisted of a muscular 

 cylinder, without cartilage, enclosing the passage to the 

 lungs. The muscular action was that of a sphincter. 



b) That the muscular cylinder is a direct derivative from the 

 system of pharyngeal constrictors, or more primitively, from 

 the muscular layer in the walls of the alimentary canal. 



c) That the arytenoid cartilages arose through chon- 

 drification of the lateral raphes, the process com- 

 mencing within the thick muscular walls and advancing toward 

 the centre. 



d) That at the moment when the growing cartilages projected 

 from the inner walls of the tube, a lever action was gained, 

 which, being of advantage to the animal, was continued until 

 it reached the stage shown in the medial portion of the larynx 



