Mil 



PHYLUM CllOKDATA 



247 



I'toiu the water, but the pectoral fins may be eini)loye(l as props 

 when it lies in a resting condition 

 at the bottom. 



External Characters. — The 



body is fish-like (Fig. 903) with a 

 diphycercal caudal fin. The surface 

 is covered with thin, bony, imbri- 

 cated cycloid scales, very kxrge on 

 the head and trunk, somewhat 

 smaller towards the tail end and 

 very much smaller over the fins and 

 the posterior part of the operculum. 



The limbs have a characteristic 

 shape, being in the form of two 

 pairs of elongated, leaf-like, pointed 

 paddles. The marginal ptirts of the 

 paired fins and the whole extent 

 of the unjjaired or caudal fin are 

 supported by a double series of 

 slender fibre-like unjointed partly 

 ossified dermal rays (^camptotrichia), 

 which are much more numerous 

 than the endoskeletal rays and 

 which are covered by small surface- 

 scales. 



The mouth is situated on the 

 ventral surface of the head, close to 

 the anterior ext,remity of the snout. 

 The external nares differ from those 

 of other Vertebrates in being 

 situated immediately outside the 

 aperture of the mouth, enclosed 

 within the upper lip. A pair of 

 internal nares opens not far behind 

 them into the anterior j^art of the 

 mouth-cavity. At the root of the 

 tail is the cloacal aperture. There 

 is an operculum similar to that of 

 the Teleostomi, with a single slit- 

 like branchial aperture behind it. 

 There are no spiracles. There is a 

 well-marked lateral line. 



Endoskeleton. — The spinal 

 column (Fig. 904) is represented by 

 a persistent notochord, enclosed 

 in a thick fibrous sheath, together with neural and ha3mal arches. 



A series of neural or basidorsai cartilages form the bases of the 



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