TELEOSTEI. 



359 



a variable number of unpaired or azygous integumentary ex- 

 pansions, which are known as the "median fins." When fully 

 developed (fig. 129), they consist of one or two fins on the 

 back the " dorsal " fins ; one or two on the ventral surface 

 the " anal " fins ; and one clothing the posterior extremity of 

 the body the " caudal" fin. The caudal fin is set vertically, 

 and not horizontally, as in the Whales and Dolphins; and in all 

 the bony fishes its form is " homocercal" that is, it consists of 

 two equal lobes, and the vertebral column is not prolonged 

 into the superior lobe. In all the median fms the fin-rays are 

 supported upon a series of dagger-shaped bones, which are 

 plunged in the flesh of the middle 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 (fig. 134, A). 



Fig. 134. A, Heart of the Angler (Lophius piscatoriiis). B, Arterial bulb of Bony 

 Pike (Lefiidosteus) cut open. C, Heart of the same, viewed externally : a. Auricle ; 

 v Ventricle ; b Arterial bulb. 



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, and is also closed by a " branchiostegal membrane," sup- 

 ported upon "branchiostegal rays." The branchiae are attached 

 to a series of bony branchial arches, which are connected in- 

 teriorly with the hyoid bone and superiorly with the skull ; and 



