2 36 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
In the vertebrates the organs of respiration are developed in more 
or less intimate connection with the cephalic portion of the digestive 
tract, just behind the cavity of the mouth. This part of the alimentary 
ABNER 
ae 
Via 
Fic. 243.—Pharyngeal region of a 
young Acanthias embryo. 8b, blood- 
vessels; c, coelomic cavities of gill arches; 
g, developing gills; gc, gill clefts; h, 
hypophysis; m, mouth; x, notochord; a, 
oculomotor nerve; oe, cesophagus; 2, 
peritoneal cavity; s, spiracular cleft; 
I-III, first to third head cavities. 
canal, which thus serves for the pass- 
age of food and for the performance 
of respiratory functions is called the 
pharynx. The organs themselves 
may take the form of gills or branchiz, 
adapted for aquatic respiration, or of 
lungs (pulmones) fitted for breathing 
air. In this connection must be con- 
sidered the cases of certain fishes, 
amphibia, and turtles whererespiration 
is effected in part by the skin, the 
pharyngeal epithelium, or the diges- 
tive tract. There are also a number 
of other structures—air bladder, thy- 
mus and thyreoid glands, etc., which 
are derived from the pharynx, though 
they are without respiratory functions. 
GILLS OR BRANCHIA. 
The typical gills or branchie are 
developed on the walls of some of the 
visceral clefts (gill or branchial 
clefts) which are formed in the sides 
of the pharynx. These clefts arise as 
paired pouches or grooves of the en- 
toderm of the pharynx (fig. 208). 
They extend laterally, pushing aside 
the mesoderm, until they reach the 
ectoderm, ectoderm and entoderm 
then fusing to a plate. This in most 
cases becomes perforated, so that the 
cavity of the pharynx is connected with the exterior by a series of 
openings (fig. 243), the clefts developing in succession from the 
cephalic end backward. 
These visceral pouches develop in all vertebrates, but in the mam- 
mals only a few or even none of them break through to the exterior. In 
