431 



The organs of the senses are visible to a certain extent from a surface 

 inspection : thus the nostrils are two apertures on each side which lead into the 

 front and hind ends of the olfactory sacs ; round the mouth there are grouped 

 eight sensitive feelers or " barbels " (not present in all fish) : the eyes, although 

 small, are evident enough, but the ears are entirely sheltered within the skull and 

 have no communication to the outside. Finally there are certain small holes and 

 slits chiefly on the head and along the lateral line of the body, which open into 

 canals containing sense-organs in the skin, and generally protected by bony 

 scales. 



The Catfish, except for a few scales of this sort chiefly situated below the orbit 

 (suborbital), is destitute of the ordinary scaly covering of a fish. Its skin is soft, 

 and slimy, there being innumerable cells in the skin constantly forming this layer 

 of mucus on the surface. But in most fish, the skin is strengthened by bony 

 scales, which may have minute teeth projecting through the surface as in the 

 Sharks and Sturgeons, or a continuous coat similar to tooth-enamel, as in the 

 bony pike, but are generally covered entirely by the soft epidermis. These scales 

 are usually either rounded (cycloid) or with a jagged hinder edge (ctenoid), fig. 

 2. Their number in longitudinal or vertical rows is often used for distinguishing 



Fig. 2 —A, Cycloid Scale from Lake Herring. B, Ctenoid Scale prom Rock Bass. 6/1. 



species, especially those that are perforated for the organs of the lateral line 

 referred to above. The formula for the number of scales is then written L. 63 (as 

 e.g. in the Common Sucker), but if it is desirable to take into account the number 

 of longitudinal rows above and below the lateral line, these are counted in an 

 oblique row fron the beginning of the dorsal fin downwards to the lateral line 

 and from that towards the ventral surface, the formula being then expressed (as 

 e.g. in the Lake Mullet) scales 6 — 42 to 48 — 5, the figures 42-48 indicating the 

 number of scales in the lateral line. 



Certain terms used in classifying fish are taken from the skeleton ; a short 

 description of the various parts of the Catfish skeleton is therefore inserted. It 

 is divided into the skeleton of the head, of the trunk, and of the paired limbs. 

 The skeleton of the head is again sub-divided into the cranium proper — which 



