riSH AND FISHEHIES. 3 



present are fins, either in pairs along the sides or in a single line above 

 and below. The skin is either (1) naked, (2) covered with scales, (3) 

 with bony plates, (4) or bony armour. With few exceptions fishes 

 propagate their species by means of eggs, and are hence called oviparous. 



Before proceeding to an explanation of the terms necessary to under- 

 stand scientific books about fishes, it may be well to refer to a plan for 

 their classification which is useful to remember. M. Louis Agassiz 

 proposed to arrange all fishes into four great cl?isses according to the 

 structure of the scales or bony covering. Thus there were 1. Placoids: 

 Without proper scales, but instead plates of enamel either large or 

 reduced to mere points : Sharks, Rays, &c. 2. Ganoids : — Scales 

 angular, bony below, enamelled above. There are few living species of 

 these, but there were many in former times. Our Australian Ceratodus 

 is one, and the Sturgeon is another. 3. Ctenoids : — Scales rough with 

 comb-like teeth at their free margins, such as in Perch, Soles. 4. 

 Cycloids : — Scales smooth without teeth at the hind margin : Salmon, 

 Mullet, Herring, Cod, &c. This classification is not adopted now, but it 

 is found useful in many ways in determining the zoological character of 

 a fish, and is moreover easily borne in mind. 



A more elaborate system of classification, and one generally adopted, 

 is that of Giinther, which divides the class Fishes into four sub-classes. 

 1. Pal/EICHthyes : Sharks, Ganoids, and Pays. 2. Teleostei, which 

 includes the majority of fishes. 3. Cyclostomata : Lampreys and a few 

 other rare genera. 4. Leptocardii. These divisions are more natural 

 than any others, but they are founded on minute details of anatomy which 

 require special knowledge to determine. Thus, Pal.eichthyes are 

 fishes with a contractile auricle to the heart ; intestine with a spiral 

 valve ; optic nerves not crossing. 2. Teleostei : — Fishes with an auricle 

 which does not contract, intestine not spiral, optic nerves not crossing. 

 Skeleton composed of bone, with vertebrae separated completely. 3. 

 Cyclostoma : — No auricle to the heart, intestine simple. Skeleton of 

 cartilage instead of bone. Only one nasal aperture. No jaws, but the 

 mouth surrounded by a circular lip. 4. Leptocardii : — Heart reduced 

 to mere pulsating sinuses in the great artery. Intestine simple. Skele- 

 ton partly cartilaginous and partly membranaceous. No skull and no 

 brain. 



We need not occupy ourselves with the two last orders, as Cyclosto- 

 mata, or Lampreys and Myxines, only include five genera and not a 

 dozen species, while the Leptocardii, or Lancelets, though distributed 

 all over the world, have not more than two species. By many it is not 

 considered a fish at all, but a separate class called Acrania, or headless. 



Thus according to this system there is little more than a division into 

 — 1. fishes with a skeleton of bone, and 2. fishes with a skeleton of 

 cartilage. Before passing to the sub-divisions of these sub-classes it will 

 be well to explain certain anatomical distinctions. Few fishermen will 

 be able to trace them, but to understand them is easily eflfected and will 

 give great insight into the physiology of fishes. 



All fishes (if we exclude the Lancelets) have red blood, and are pro- 

 vided with a complete circulation for the body, another for the gills, and 

 a third for the liver. But fishes difter from other animals in this, that 

 the heart is relatively small, and is provided with only two chambers, 



