488 ANATOMY OF VERTEBRATES. 



second and third arches, is such that water can with difficulty 

 penetrate them, and they are usually found to contain air. They 

 are not, however, the homologucs of the air-bladder or of lungs, 

 though they are analogous to the latter in function. By this 

 extreme modification of the opercular gill the Singio (Sacco- 

 branchus, Cuv.) is enabled to travel on land to a great distance 

 from its native rivers or marshes, and, like the Cuchia, is remark- 

 able for surviving the infliction of severe wounds. 1 In most fishes 

 a rich developement of follicles on the Avails of the gill-chamber 

 supplies the branchial machinery with a lubricating mucus. 



The mechanism of branchial respiration differs from that of 

 swallowing, only in the streams of water being prevented from 

 entering; the smllet, anc { beino; diverted to the branchial slits on 

 each side the pharynx. 



The mouth opens by the retraction of the premaxillary and the 

 depression of the mandible. Almost simultaneously the mandi- 

 bular rami are divaricated behind by the action of the ' levatores 

 tympani,' fig. 134, 24, upon their pedicles ; the opercular flaps are 

 drawn outward by the 'levatores operculi,' ib. 25 ; the branchio- 

 stegal membrane is dilated by divarication of the rays, the ' leva- 

 tores branchiostegarum,' fig. 135, 28, opposing the ' depressores,' 

 ib. d, in this action ; the branchial arches are successively drawn 

 forward and outward by the ' branchi-levatores,' fig. 137,3, and 

 ' mastobranchiales,' ib. 26 ; and the branchial chamber being thus 

 expanded, the water rushes in through the sieve-like inner slits, 

 and fills the chambers, floating apart the gills and filtering 

 between every branchial process and fold. The inner slits are, 

 then, closed by the protraction of the hyoid and depression of the 

 branchial arches, the ' geniohyoidei,' fig. 135, cooperating with 

 the ' branchi-depressores,' fig. 137, £5, in this action ; the branchial 

 processes are approximated and divaricated by special muscles, 

 and elastic parts. The respiratory currents are driven out by the 

 contraction of the branchiostegal membranes and the depression 

 and adduction of the opercular flaps, which, on the expulsion of 

 the currents, close like a door upon the ' sill ' formed by the 

 scapular arch. In the Plagiostomes the branchial currents are 

 moved and directed by muscles, combined with elastic structures, 

 more immediately acting on the inner and outer slits and the 

 intermediate chambers. 



§ 85. Arteries of Fishes. — The first structure to be noticed in 

 connection with the arterial system, is the vascular body already 

 alluded to under the name of ' pseudobranchia.' Mormyrus, Tinea, 



1 CXVII. 



