90 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



1. Elasmobranchii (sharks and rays). In 

 " cartilaginous fishes " the gill slits are, in the simples^ 

 naked, i.e. not covered over by any fold (opercnluni), 

 neither the skull nor the jaws are ever protected by 

 ossifications of the investing membrane (membrane 

 bones) ; the notochord has the outer sheath pro- 

 vided with rings of ossification, or distinct vertebrae 

 become developed. The skin is either naked, or 

 covered with calcified tooth-like papillae. 



2. Dipnoi; e.g. Lepidosiren, Ceratodus. In 

 these the cartilaginous brain capsule becomes invested 

 by bones developed in the covering membrane, and 

 the digestive tract gives off a single or incompletely 

 divided air sac, which is more or less richly supplied 

 with blood-vessels, and may undertake the office of a 

 lung, the possession of which enables the fish to live 

 in mud. The pectoral and pelvic tins are broad and 

 paddle-like (Fig. 45), or elongated and filiform. 



3. The Ganoidei and (4) Teleostei are the 

 two groups of the Pisces in which we observe that 

 elaboration of the details to which reference has 

 already been made; a cod, a sole, or an eel stand 

 almost as far from the primitive vertebrate as the 

 snake, the hawk, or the bat. The former group 

 retains certain more primitive characters which are 

 only rarely or rudimentarily possessed by the latter ; 

 thus the arterial trunk (see page 195), which is 

 muscular and contractile in Ela^mobranchs, Dipnoi, 

 and Amphibians, is so also in Ganoids, but is only 

 incompletely so in some Teleostei (Butirinus) ; the 

 spiral valve which is found in the intestine of 

 Elasmobranchs is retained in the Ganoids, though not 

 well developed in the Sturgeon and its nearest allies ; 

 it is lost in most Teleostei, though found in Butirinus 

 (Stannius), in Chirocentrus, and perhaps represented 

 in rudiment in the smelt (Huxley). 



In both groups the ends of the gills are free, and 



