502 



CHORDATA. 



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

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

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

 would be exposed to serious injure' were ihev not protected by the opcr- 

 cidum (op) or gill co\-er. This is a fold of skin arising from the hyoid arch 

 and extending back o\'er the gill region. It is supported by two groups 

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

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

 latter supporting the hrainiiioslcgal mcmhraiw. Between the free edges of 

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

 beliind is the opercular clef I (lig. 551, ops), wliich is 

 obviously not identical with a gill cleft, but leads into 

 an alrium into which the gill clefts empty. 



In many elasmobranchs and ganoids there is a 

 rudimentary cleft, the spiracle, between the ptervgoquad- 

 rate and hyomandibular, in which a rudimentary gill, 

 or pscudobranch, 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 larvx, a matter in which 

 the two groups were thought to differ. This explains 

 the existence of external gills in Protoptcrus and the 

 larv;e ol Lcpidosireii and Polyptcriis. 



Besides gills, tishes, \\1th the exception of 

 elasmobranchs and some teleosts, have a sicim 



55^. — Sections 

 arcfies of Ltudus 



hl^ 



(lefO and Zv^o-Ho (right), Madder, usually regarded as the homologue of the 



lungs. It is often shaped like an hour glass, filled 

 with air, and may open into the a^sophagus by a 

 pneiiiiijlic duct (Physostomi), or this, appearing in 

 development, may be lost in the adult (Ph\'soclisti). 



slightly enlargcrh 

 artery; />, gill arch; 

 hi'-, demibranchs;/;, der- 

 mal projection; r, carti- 

 lage rav;?', vein;c, tooth. 



In the physoclisti there is a spot, the 'oval,' 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. \\'hen a 

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

 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's^ases from the 

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

 spots' or 'gas glands,' 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 onlv as long known in the Dipnoi, but in 

 other forms like Lepidostciis and Ainia. 



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 



