484 MANUAL OF ZOOLOGY. 



median fins the fin-rays are supported upon a series of dagger- 

 shaped bones, which are plunged in the flesh of the middle 



Fig. 261. The common Perch (Perca Jluviatilis). o Gill-cover, with the gill-slit be- 

 hind it ; / One of the pectoral fins, the left ; i> The left ventral fin ; ^The first dorsal 

 fin ; d' The second dorsal fin ; c The caudal fin or tail ; a The anal fin ; / Lateral line. 



line of the body, and are attached to the spinous processes of 

 the vertebrae. These are the so-called " interspinous " bones. 



IV. The heart consists of two chambers, an auricle and a 

 ventricle, and the branchial artery is furnished with a bulbus 

 arteriosus. The arterial bulb, however, is not furnished with a 

 special coat of striated muscular fibres, is not rhythmically con- 

 tractile, and is separated from the ventricle by no more than a 

 single row of valves. 



V. The respiratory organs consist of free, pectinated, or tufted 

 branchiae, situated in two branchial chambers, each of which 

 communicates internally with the pharynx by a series of clefts, 

 and opens externally on the side of the neck by a single aper- 

 ture (or " gill-slit "), which is protected in front by a bony gill- 

 cover (fig. 261) and is also closed by a " branchiostegal mem- 

 brane," supported upon " branchiostegal rays." The branchiae 

 are attached to a series of bony branchial arches (generally 

 five on each side, but only the anterior four bearing gills), 

 which are connected inferiorly with the hyoid bone, and 

 superiorly with the skull ; and the water required in respira- 

 tion is taken in at the mouth by a process analogous to swal- 

 lowing. 



"hypural bone," to the free edges of which the caudal fin-rays are sym- 

 metrically attached. The actual termination of the notochord is bent up, 

 and is never ossified ; but its sheath usually becomes calcified, forming a 

 spine ("urostyle") which coalesces with the dorsal edge of the hypural 

 bone, the latter being formed by the anchylosis of ossicles developed from 

 the ventral face of the notochordal sheath. 



