382 MANUAL OF ZOOLOGY. 



I. The skeleton, instead of remaining throughout life more 

 or less completely cartilaginous, is now always more or less 

 thoroughlyossified. The notochord is not persistent, and the 

 vertebral column, though sometimes cartilaginous, consists of a 

 number of vertebrae. The bodies of the vertebra are what is 

 called " amphiccelous " — that is to say, they are concave at 

 both ends. It follows from this, that between each pair of 

 vertebrae there is formed a doubly-conical cavity, and this is 

 filled with the cartilaginous or semi-'gelatinous remains of the 

 notochord. By this means an extraordinary amount of flexi- 

 bility is given to the entire vertebral column. In no fish except 

 the Bony Pike (which belongs to the order Ganoidei) is the 

 ossification of the vertebral centra carried further than this. 

 The skull is of an extremely complicated nature, being com- 

 jDOsed of a number of distinct cranial bones j and a mandible 

 or lower jaw is invariably present. 



II. The anterior and posterior pairs of limbs are usually, but 

 not always, present, and when developed they are always in 

 the form of fins. The fins may be supported by " spinous " or 

 " soft " rays, of which the former are simple undivided spines 

 of bone, whilst the latter are divided transversely into a num- 

 ber of short transverse pieces, and also are broken up into a 

 number of longitudinal rays proceeding from a common 

 root. (The Fishes with soft rays in their paired fins are 

 termed "Malacopterygii" — those with spinous rays, " Acan- 

 thopterygii.") 



III. Besides the paired fins, representing the limbs, there is 

 a variable number of unpaired or azygous integumentary ex- 

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

 developed (fig. 137), 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 isnot prolonged 

 into the superior lobe. In all the median fins 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 rhytlimically con- 



