«S4 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



respiratory in character, but, in some cases at least, they also serve 

 for the absorption of food from the egg or the uterine fluids. 



Fig. 272. — Diagram of relations of oesophagus and respiratory tracts in (^4) Myxine 

 and Ammocoetes, and {B) Peirotnyzon: b, branchial duct ('bronchus'); oe, oesophagus; 

 /, thyreoid gland. 



In the cyclostomes and notidanid sharks the first cleft (between 

 the mandibular and hyoid arches) bears gills like the rest, but else- 

 _ where it differs. In most elasmobranchs and a 



few ganoids (sturgeon, Polypterus) it becomes 

 reduced in size by closure beginning below (fig. 

 143) so that the persisting part of the original 

 opening is on the dorsal side of the head, form- 

 ing the spiracle. In other vertebrates, includ- 

 ing the chimaeroid and many true sharks, the 

 spiracle is closed in the adult, but in the anurous 

 amphibia and the amniotes its inner portion 

 persists as the tympanic cavity and the Eusta- 

 chian tube of the ear (p. 202). 



Usually the series of gills begins with the 

 demibranch on the posterior face of the hyoid 

 arch, but there is never a demibranch on the 

 posterior wall of the last cleft. In some cases 

 the series of gills is still farther reduced by the 

 loss of demibranchs from other arches, the re- 

 duction reaching the extreme in the symbran- 

 pouches and blood-vessels chiate genus Amphipnous, where there are no 

 of Myxine, a.ittx umci. g{\\^ on the first and fourth branchial arches 



by gill pouches; ed, effer- ° , ., 1 i ^ 



ent ducts; eo, external gill and only one demibranch on the second. 



opening; h, heart; oc, 



cesophageo-c utaneous In cyclostomes the gill clefts in the adult are some 

 ventraUort?!'^'^^''''' "''' distance behind the mouth, partly as the result of the 

 great development of the lingual apparatus. In the 

 larvae of Peiromyzon (Ammocoetes) the seven clefts are nearly typical, the 

 demibranchs extending inward nearly to the pharyngeal wall, each cleft having 

 a short efferent duct leading to the exterior, while the oesophagus begins at the 

 hinder end of the respiratory region (fig. 272, A). With the metamorphosis 



