22 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



occur, a sublingual, a submaxillary, and a parotid. None of 

 these are poisonous ; but the saliva which they secrete is for 

 moistening the food during mastication, and for the conver- 

 sion of starch into sugar. 



From the pharyngeal region are devel- 

 oped the respiratory organs, gills and 

 lungs, as well as certain other struc- 

 tures, the thyroid gland, thymus gland, 

 etc. 



Gills. Gills arise as a series of paired 

 or bilateral outpushings of the entodermal 

 lining of the pharynx. These push out 

 through the mesodermal and mesenchyma- 

 tous tissues until they reach the ectoderm 

 on the sides of the neck. The two layers 

 now fuse, and then an opening is formed 

 at the point of fusion, so that there arise a 

 series of openings (gill-, branchial-, or vis- 

 ceral-clefts) on either side, connecting the 

 pharyngeal cavity with the external world. 

 In the septa between the clefts are devel- 

 oped skeletal structures (gill- or branchial- 

 arches, see skeleton), and also blood-ves- 

 sels. From the walls of the clefts develop 

 vascular leaves or filaments, the gills proper. 

 These are arranged on the anterior and pos- 

 terior walls of the clefts, those on a side 

 constituting a demibranch. 



The number of gill pouches differ in 

 different groups. In Bdellostoma a (cy- 

 clostome) there may be 14 pairs ; in the 

 notidanid sharks 7 or 8 ; in other sharks 6, 

 and from this down to 5 in reptiles, and 4 in mammals. In the 

 ichthyopsida all, or nearly all, of these pouches break through 

 as described above, but in the amniotes but one or two open 

 to the exterior ; the statements regarding the mammals being 

 conflicting. In the amniotes these gill pouches or clefts never 

 develop gill filaments ; and in the adult all traces of them, except 



FIG. 



Horizon- 



tal section through head 

 and pharyngeal region of 

 Acanthias embryo, show- 

 ing the gill slits. b, 

 blood-vessels; r, coelomic 

 cavities of gill arches ; 

 /, developing gill fila- 

 ments; //, hypophysis; 

 n, notochord ; 0, oculo- 

 motor nerve ; />, pharynx ; 

 s, spiracular cleft; /, first 

 (mandibular) head cav- 

 ity ; /- T, gill clefts. 



