MARINE VERTEBRATES 339 



White Salmon and the Salmon Dace. This fish may be taken with 

 rod and line on rocky coasts and at the mouths of rivers. The sand- 

 eel, or an artificial imitation of it, is commonly used as bait, but 

 the Cornish fishermen more frequently employ a piece of herring or 

 pilchard for the purpose. The first dorsal fin of this fish has very 

 strong spines which may inflict severe wounds when the live crea- 

 ture is carelessly handled. 



Omitting all mention of sea birds, for the reason previously 

 given, we now pass to the highest division of vertebrates the 

 Mammals of which we shall describe but one species the Com- 

 mon Porpoise, this being the only marine mammal that can- be 

 regarded as a frequent visitor to the British coasts in general. 



It may be well at the outset to understand exactly why the 

 porpoise is classed with the mammals and not with the fishes to 

 see how its structure and functions correspond with those of oui 

 own bodies rather than with those of the animals dealt with in the 

 preceding portion of the present chapter. 



First, then, while the young of fishes are almost invariably pro- 

 duced from eggs and are not nourished by the parents, the young 

 of the porpoise are produced alive, and are nourished with milk 

 secreted by the mammary glands of the mother. This is an all- 

 important feature, and is the one implied in the term mammal. 

 The porpoise also differs from nearly all fishes in that it breathes by 

 lungs instead of gills, obtaining its air direct from the atmosphere, 

 and not from the water. Hence we find it coming to the surface at 

 frequent intervals to discharge its vitiated air and to inhale a fresh 

 supply. The body-cavity of a mammal is divided into two parts by 

 a muscular diaphragm, the foremost division, called the thorax, con- 

 taining the heart and lungs, and the other (the abdomen) the 

 remaindsr of the internal organs, while the diaphragm itself plays 

 an important part in the respiratory movement by which air is 

 drawn into the lungs. The body of the porpoise is so divided, but 

 no such division ever occurs in any of the fishes. Lastly, the heart 

 of the porpoise, in common with the rest of the mammals, is divided 

 into four cavities, and the blood is warm, while the heart of a fish 

 has generally only two divisions, and the blood propelled by it is of 

 about the same temperature as that of the surrounding medium. 

 Several other important differences between the porpoise and the 

 fish might be given, but the above will be quite sufficient to show 

 why they are placed in different classes. 



