240 



ZOOLOGY. 



VC3/CL 



14. History of the Gills and Related Structures. At 

 the time of hatching the first two branchial arches show at 

 the sides of the head as slight vertical ridges (fig. 122 [1]). 

 From these there soon grow out long branching processes, the 

 external gills. These are, of course, covered by epiblast, and 

 have a core of mesoblast, containing a series of looping 



blood - vessels 

 which con- 

 nects in each 

 case the af- 

 ferent and 

 efferent ar- 

 teries of the 

 respective 

 arch. Pre- 

 sently a 

 similar ex- 

 ternal gill 

 grows out 

 from the 

 third bran- 

 chial arch of 

 either side 

 (fig. 123, left 

 side). 



M e a n - 

 while, the 

 hypoblast of 

 the pharynx 

 been 

 its 



way towards 



the surface between all the visceral arches ; and these 

 outpushings of hypoblast at length unite with the epiblast, 

 and the gill-slits are formed. These are four pairs, for 

 the one between the mandibular and hyoid arches (answer- 

 ing to the dogfish's spiracle) never becomes perforated ; 

 and there is neither slit nor arch answering to the fifth 

 of the dogfish. These gill-slits are perforated about the 

 same time as the mouth, so that feeding ard throat- 



Fig. 128. DIAGRAMMATIC HORIZONTAL SECTION OF TADPOLE, 



Showing a later stage on the right side than on the left. The h a S 

 black spots in the arches represent cartilage, and the white 

 circles, arteries. (Partly after Marshall.) 



