544 COMPARATIVE ANATOMY. 



consequently placed in two rows on eacli arch as it passes between 

 two branchial pouches (Fig. 306, B b). The anterior row of branchial 

 lamellae in a Teleostean or a Ganoid corresponds, therefore, to the 

 gill on the hinder wall of the branchial pouch of a Selachian, and the 

 hinder row of lamella? in a Teleostean gill corresponds to the anterior 

 gill in the branchial pouch of a Selachian. 



These relations are shown in the following diagram, in which b 

 is the indifferent stage of the rows of branchial lamella?, B is the 

 differentiated arrangement in the various divisions. /3 represents any 

 row of branchial lamella?, which is specially modified or reduced : 



Selachii: /3 B l B 2 B^ B* B> 



Ganoidei fibbbbbbbbb 



and Teleostei : — fi' B l B 2 B* B* 



By the degeneration of the septa between the branchial pouches, 

 the whole gill apparatus is made more compact, and no longer 

 therefore extends back into the region of the trunk, as it does in the 

 Selachii ; it is confined to the base of the skull. Whilst in the 

 Selachii the projecting septum (A s) forms an organ of protection 

 for the succeeding branchial pouch, a similar organ is formed in 

 the Chimaera?, Ganoidei, and Teleostei, from a single arch — namely, 

 from the hyoid ; the integument on this arch grows backwards and 

 covers all the gills, and is developed, in the Ganoidei and Teleostei, 

 into the opercular apparatus and the branchiostegal membrane, with 

 their various skeletal pieces (§ 354) (B op). 



§ 408. 



In the Teleostei four arches are ordinarily beset with branchial 

 lamella?, the fourth arch having a single row only, or there are but 

 three arches which carry lamella?. When the lamella? of the fourth 

 arch, and the posterior row on the third arch disappear, the fourth 

 branchial cleft is ordinarily closed. Perhaps one of the most im- 

 portant of the modifications which affect the lamella? themselves is 

 seen in the villous gills of the Lophobranchiata. In some divisions of 

 the Teleostei, the branchial arches seem to be so metamorphosed as 

 to be able to retain the water in the branchial apparatus. The 

 organs of the Labyrinthobranchiata arc of this kind; separate 

 branchial arches or parts of such, are modified, to form coiled lamella- 

 like processes, which give rise to a portion which is placed above the 

 gills (Anabas, Poly acanthus). Another apparatus, which is found in 

 various Clupeida?, consists of a spirally-coiled tube (branchial coil), 

 which is formed by a diverticulum of the superior pharyngeal mucous 

 membrane. This tube is generally connected with the superior 

 segment of the fourth branchial arch, and has processes of its 

 skeletal parts in its walls (Heterotis, Lutodeira, Meletta, etc.). The 



