ee Be yaks  —— —— a 
- 2 431 
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 chiefty 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 
 seales. 
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 
B. 
Fig. 2.—A, Cyctomp ScaLE FROM LAKE HERRING. B, CrENoID ScALE FROM 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), butif it is desirable to take into account the number 
ot 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 
we Le ve 
Se rr Se ere or ee oe eres | 
