PISCES. 153 



1. Cydostomata lampreys and hags. 



2. Elasmolmnchii dogfish, shark, skate. 



3. Ganoidei sturgeon. 



4. Teleostei herring, perch, cod, eel. 



5. Dipnoi mudfishes. 



The last group is a small and highly specialized one, including a 

 few fresh-water forms which breathe by gills and also by a lung- 

 like swim-bladder. It includes Ceratodus, in Australia ; Protopterus, 

 in Africa ; and Lepidosiren, in S. America. 



The large majority of recent fishes belong to the Teleostei, some 

 of the most obvious characteristics of which are an externally 

 symmetrical tail, terminal mouth, thin scales, comb-like gills 

 protected by a firm gill-cover, and a well-ossified endoskeleton. 



The Ganoidei are represented at the present time by a small 

 number of genera widely distributed in the fresh waters and 

 estuaries of the globe. The group was once large and important, 

 but is now approaching extinction. The recent Ganoids, of which 

 the best known is the one (Acipenser) including the sturgeon, 

 form a very heterogeneous assemblage, not closely related, and 

 difficult to include in a common definition. 



Cydostomata are limbless, jawless fishes with a suctorial mouth. 



Elasmobranchs are a very ancient type, and though common at 

 the present day are relatively far less abundant than they were 

 in former geological epochs. The skates are a good deal special- 

 ized, but dogfishes and sharks present the features of the fish- 

 type in a comparatively unmodified condition, and are, therefore, 

 better general illustrations of the group of fishes than members 

 of the teleostei. Dogfishes, which may be regarded as small 

 sharks, are represented on the British coasts by several genera, of 

 which Scyllium, the Spotted Dogfish, is perhaps the commonest. 

 S. canicula, the one usually dissected in laboratories, has an 

 average length of about two feet; S. catulus is much larger. 

 The following account will apply to either. 



14. SCYLLIUM (Dogfish). 

 MORPHOLOGY AND PHYSIOLOGY. 



1. External Characters. The elongated spindle-shaped body, 

 eminently adapted for rapid progression through water, exhibits 



