THE EXCRETORY ORGANS. 



475 



AND TPPER PART 

 OF TRACHEA OF 



dus). Front (i.e. 

 ventral) aspect. 



(After He ule.) 

 , epiglottis ; f and 

 g, muscles of la- 

 rynx. 



cricoid cartilages may be represented by a single circular 

 cartilage with arytenoid cartilages annexed, as in the Croco- 

 dile. This circular cartilage may present defects of solidi- 

 fication like those just spoken of in the thyroid of Birds, as 

 is the case in many Lizards. An essentially 

 similar structure may be greatly prolonged, 

 as in many Serpents (e.g. Crotalus and Hydro- 

 phis'], where we find two elongated lateral 

 bands connected by numerous transverse 

 bars which are separated from each other 

 by membranous interspaces. Arytenoid 

 FIG - HRYNX cartu<a & es ma y be distinct or may be mere 

 processes. 



The simplest form of larynx seems to be 

 that of Proteus, where there are only two 

 slender elongated cartilages (the anterior 

 ends of which appear to represent the aryte- 

 noids) placed one on each side of the slit- 

 like glottis, which opens in the middle of the 

 ventral wall of the pharynx. 



A median air-sac may protrude anteriorly 

 from the front of the larynx 

 between the first ring of the 

 trachea and the conjoined thy- 

 ro-cricoid cartilage. Such is 

 the case in the Chameleon. 



8. As has been said, respi- 

 ration may be effected by 

 means of the air contained in 

 water i.e. by AQUATIC RESPI- 

 RATION. For this purpose the 

 animals which so respire do 

 not ordinarily take water into 

 aquatic lungs as the air-breath- 

 ing Vertebrates take air ; but 

 blood is copiously brought to 

 a surface specially exposed to a 

 flow of water almost constantly 

 maintained by one process or 

 another. 



It may be that this flow is 

 effected by the action of vibratile cilia 1 propelling the water 

 through numerous perforations in a greatly enlarged pharynx, 



1 For a description of cilia see " Elementary Physiology," Lesson VII. 3- 



FIG. 400. TKACHBAL STRUCTURE 

 OF Proteiis, opened from behind. 



A, arytenoid part of each of the two 

 cartilages ; T, its tracheal part ; />', 

 branchial arch ; M, muscle going 

 from the branchial arch to the 

 larynx. 



