430 



of respiration. The body is divisible into three regions, head, trunk and tail 

 which have different duties to discharge, and consequently differ m torm and 

 structure The head lodges the brain and sense organs, secures food and shelteis 

 the oills ; the tail is chiefly locomotive in function, while the trunk^ differs from 

 botlfin 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 

 surfaceas 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, i.e., bony or 

 spine-like in their whole length, or else fringed and jointed. For the purpose 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, I, 6. 



Anal, 22. 



Pectoral, * ^ I, 6. 



Ventral, I, 7. 



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 ot 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. 



