/I'. VKllTEVIiATA: PISCES. 



:,Cu 



Fig. o'.tr). — Sc'OtioTis of u'iU 

 arches i)f Liiliht.^ (Ifft) 

 and Ziiiji'-iin (rit,')it), 

 slit;litly enlarged, n, 

 artei-y ; '', gill areh ; 

 hi', hi-, deinibraiiolis ; 

 //, dermal pro.jection; 

 /■, cartilage ray ; r, 

 \'ein : z, toutli. 



(j7jr) from the byoid, these latter svipiiortiiig the bnuiehiostega] 

 niembvaiie. ]>et\veen the free edge of the 

 (jjiercuhini and the liraneliio.stegal mem- 

 brane and tlie skii^i of tlie body Ijehiud i.s 

 tlie opereular cleft (fig. 5'.)-l, opy), ^vhi(:■h h 

 obviously not identical with a gill cleft, but 

 leads into an atrinm into "which tlie gill 

 clefts empty. In many elasmobraiichs and 

 ganoids there is a rudimentary cleft, the 

 spiracle, between the pterygoquadrate and 

 hyomandibular, in v\hich a rudimentary gill, 

 or pseudobranch, may occur, this often j'cr- 

 sisting when the spiracle is (dosed. 



Besides gills, fishes, ^\]t]\ the exception 

 of elasmobraiichs and some teleosts, have a 

 swim bladder which is usually regarded as 

 the homologue of the lungs. It is often 

 shaped like an hour glass, filled with air, and 

 may open into the O'soplnigus liy a pneuniatie 

 duct (Physostomi), or this, appearing in de\elo})ment, may be lost 

 in the adult (Pliysoclisti). 'J'lie air bladder serves fur respiration 

 in the Dipnoi and possibly in some ganoids {Lcjiidoslnis and 

 Amla), but is usually a hydnistatie ap})aratus, its enlargement or 

 compression altering the specific gravity of the fish. In fishes 

 brotight uj) from great depths the e.\j)a.nsion of air in tlie swim 

 bhidder frequently forces the viscera. (Hit thi'duali the mouth. 



1'he heart, encloseil in the jierienrdiuni, lies immediately 

 behind the gill region, and is }ir(ilc<'ted by the shoulder girdle. 

 It always consists of auricle and \'eiitricle (fig. •")'.»<;), se})arated by 

 a. jiair of valves to prevent liaeb-llow of the bhiod; it sends the 

 Idood to the gills by the arterial tniid< (ventral aorta), and receives 

 it from the body through a thin-\\-alled sac, the \('nous simis, in 

 whitdi the hejiatic veins anil the ('m'ierian ducts (formed by union 

 of jugular and cardinal veins) empty (figs. 0,5, ."i!)7). 



''I'lie most important diifereni'es lie in the devi'lopment of eomis 

 and biilbus arteriosus. These are nmsi-ular accessor\' oi'gaiis, the 

 lii'st arising from the heart, the otlier from the arterial trunk; and 

 correspondingly the conns has striped, the tmlbtis sniocjth mttscle 

 fibres. The anterior end of tlie heart contains ' semilunar ' valves, 

 «hicli, like the auriinihi-ventviciilar valves, prevent the back-fiow 

 111' the bliiod. ^\'hen, by increase in the numlier of valves, this part 

 1 (unes idongate, afdUiis arteriosus (fig. .j'.iil, ,1) is formed. The 



