430 La 
= Ae “ 
of respiration. The body 1 is divisible into three regions, head, trunk and Pe is a | 
which have different duties to discharge, and consequently differ in form and a 
structure. The head lodges the brain and sense organs, secures food and shelters 
the gills; the tail is chiefly locomotive in function, while the trunk differs from 
both in being hollowed out so as to enclose the intestines and other viscera in the © 
so-called body-cavity. From the trunk there project the two pairs of limbs or 
members corresponding to our arms and legs, but which are here called on account 
of their position and form the pectoral and ventral “ fins.” 
These must be carefully distinguished as the “paired” fins, from the 
“unpaired ” fins, which occupy the middle line of the trunk and tail, sometimes 
forming, in other fishes, a continuous fold as far forward on the under or ventral 
surface as the end of the intestine, but more frequently interrupted as in this 
species into dorsal, caudal and anal fins. There are in fact two dorsal fins in 
the Catfish, but the hinder of the two, instead of being supported by fin- “rays, only 
includes within it some fatty tissue and is therefore called the “ adipose” fin. * 
The fin-rays which support the fins are either hard or soft, ve. bony or 
spine-like in their whole length, or else fringed and jointed. For the purpuse of 
distinguishing different species it is often desirable to count the number of rays 
in the various fins, and express them in a formula, using Roman numerals for the 
hard, and Arabic for the soft rays. 
Thus, for this species, the formula is: 
Dorsal, : = = ae oy 
Anal, - - 22: 
Pectoral, - : = a eg 
Ventral’). + : - i Faw 
Certain apertures are to be noted, viz.: the mouth bounded by the upper 
and lower jaws and leading into the cavity of the mouth, which opens behind 
into the gullet and at the sides and floor by the gill-slits (five in number on each side) 
into the right and left gill or branchial chambers. The opening into the gullet is 
occupied by tooth-bearing bony plates above and below, the superior and inferior 
pharyngeal plates, and the inner opening of the gill-slits which are separated by 
the “ gill-arches,” are screened by a series of short projections on the concave 
surfaces of the gill-arches, those looking into the cavity of the mouth. The pro- 
jections, which are often in other fish of considerable length, are called “gill- 
rakers,” and serve to strain the water that flows out over the gills. 
The latter occupy the gill-chambers and are attached in two rows to the outer 
convex faces of the four gill-arches. They are concealed by the gill-cover, a flap 
which bounds the gill-chamber externally, and the free margin of which can be 
applied tightly against the shoulder-girdle, the bones of which strengthen the 
- wall of the gill-chamber below and behind. Supporting the main part of the gill- 
cover are the “ opercular ” bones, while a series of “branchiostegal rays” strengthen 
its lower free margin. Between the gill-openings on the ventral surface is the 
narrow unperforated floor of the mouth, which in many fishes forms a very narrow 
“isthmus.” 
The intestine opens posteriorly by a vent or anus in front of the anal fin, 
_ Behind the anus are the openings of the reproductive and urinary organs, separate 
in the females, but on a common urogenital papilla in the males. ‘, 
