488 ANATOMY OF VERTEBRATES. 



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

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

 are not, however, the hoiuologues 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- 

 branclius, 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.' In most fishes 

 a rich developemcnt of follicles on the walls of the gill-chamber 

 supplies the branchial machinery with a lubricating mucus. 



The mechanism of branchial resj^iration differs from that of 

 swallowing, only in the streams of water Ijcing jirevented from 

 entering the gullet, and being diverted to the branchial slits on 

 each side the pharynx. 



The mouth opens by the retraction of the prcmaxillary and the 

 depression of the mandible. Almost simultaneously the mandi- 

 Ijiilar rami are divaricated behind by the action of tire ' levatores 

 tympani,' fig. 134, 24, upon their pedicles; the opercular flajxs 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. 1.35, 28, opposing the ' depressorcs,' 

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

 forward and outward by the ' branchi-levatorcs,' 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 filteriug 

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

 then, closed ))y the protraction of the hyoid and depression of the 

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

 the ' branclii-dcpressorcs,' fig. 137, f 5, in this action ; the branchial 

 2)roccssos arc approximated and divaricated liy special muscles, 

 and elastic parts. The respiratory currents are driven out liy 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 

 sca[iular arch. In the Plagiostomes the branchial currents are 

 moved and directed by muscles, combined with elastic structures, 

 more inunediately acting on the inner and outer slits and the 

 intermediate clianfljcrs. 



§ 85. Arteries nf Fishes. — Tlic first structure to be n<iticcd in 

 connection with the arterial system, is the \'ascular body already 

 alluded to under tlienamcof' [iscudobranchia.' JLiniii/riis, Tinea, 



