502 



CHORDATA. 



or less completely disappeared, so that the demibranchs of one arch 

 become connected, their free ends projecting into the water like the teeth 

 of a double comb. Here, on account of their very delicate structure, they 

 would be exposed to serious injury were they not protected by the oper- 

 culum (op) or gill cover. This is a fold of skin arising from the hyoid arch 

 and extending back over the gill region. It is supported by two groups 

 of bones, the opercular bones proper (fig. 547, O, So, lo, Pro), attached to 

 the hyomandibular, and the branchiostegals (rbr) from the hyoid, these 

 latter supporting the branc/iiostegal membrane. Between the free edges of 

 the operculum and the branchiostegal membrane and the skin of the body 



behind is the opercular cleft (fig. 551, ops), which is 

 obviously not identical with a gill cleft, but leads into 

 an atrium into which the gill clefts empty. 



In many elasmobranchs and ganoids there is a 

 rudimentary cleft, the spiracle, between the pterygoquad- 

 rate and hyomandibular, in which a rudimentary gill, 

 or pseudobranch, may occur, this often persisting when 

 the spiracle is closed. The gills proper develop from 

 that part of the cleft derived from the skin, they are 

 therefore ectodermal in origin, agreeing with the exter- 

 nal gills of the amphibian larvae, a matter in which 

 the two groups were thought to differ. This explains 

 the existence of external gills in Protopterns and the 

 larvae oiLepidosircn and Polypterus. 



L^~-^ 



of 



Besides gills, fishes, with the exception of 



FIG. 552. Sections elasmobranchs and some teleosts, have a swim 

 of gill arches of Gadus .. 



(left) and Zygana (right), bladder, usually regarded as the homologue of the 



slightly enlarged, a, l ung s. It is often shaped like an hour glass, filled 



artery; b, gill arch; W, . . 



bl*, demibranchs ;Mer- Wlth air > and ma 7 P en mto the oesophagus by a 



mal projection; r, card- pneumatic duct (Physostomi), or this, appearing in 

 lageray;^, vein; s, tooth. . 



development, may be lost in the adult (Physochsti). 



In the physoclisti there is a spot, the 'oval, 1 in the region of which is a richly 

 vascular network. Apparently this is for the resorption of the gases of the 

 bladder when the pressure is reduced in going to a higher level, a matter accom- 

 plished in the physostomes by passage of the gases through the duct. When a 

 fish is rapidly brought to the surface from great depths, neither process is suffi- 

 cient, and in order to accommodate the expansion of the bladder, the viscera are 

 frequently forced from the mouth. As the fish can resorb the gases from the 

 swim bladder, it can reform them, they being secreted from the blood the 'red 

 spots' or 'gas ,t,'/</;/</\,' spots rich in blood-vessels and covered with a special 

 epithelium. The possibility of this gas exchange shows how the swim bladder 

 can function as a respiratory organ, not only as long known in the Dipnoi, but in 

 other forms like Lcpidosteus and Amia. 



Regarding the chief functions of the swim bladder there are two views which 

 arc not incompatible, (i) The swim bladder is a hydrostatic apparatus, since 

 the ability to regulate the amount of the contents makes it possible to compensate 



