435 



each other in the middle line below, but each half at an early date is formed "of 

 two elements, a primary element formed in cartilage, and a secondary larger one 

 formed beneath the skin. The so-called basal elements of the pectoral fin corres- 

 pond in part to the limb bones of higher animals, but they are unimportant in size 

 as compared with the fin-rays, the foremost of which can be set and used as an 

 effective weapon by the catfish. 



Systematic zoologists rarely employ the form of the viscera to any extent 

 in framing comparative descriptions of different species. The following points 

 as to the arrangement of the viscera should, however, be noted. 



The intestinal canal is a comparatively short tube which is marked* off into 

 three regions, the stomach, small intestine and large intestine. All these parts are 

 supported to the wall of the body-cavity in which they lie by the mesentery, and 

 another part of the same membrane the " peritoneum " also lines the body-cavity. 

 Its colour may vary from black to silvery. Connected either permanently, or in 

 the young fish merely temporarily, with the gullet is the air-bladder, a part of the 

 intestine which serves as a float, and may have other subsidiary functions. 



The stomach may merely be a slightly enlarged part of the intestinal tube, 

 but may have a blind projection as in the catfish, ' coecal type," or be bent into 

 U-shaped form, " siphonal type." It is separated from the first part of the small 

 intestine by a muscular valve, the pylorus, from which a number of blind tubes 

 attached to this part of the intestine in many fishes (but not the catfish) derive 

 the name of pyloric coeca. Opening into this part of the intestine also is the 

 bulky liver, which is provided with a large gall-bladder. 



The heart in the catfish is situated far forwards in the region of the throat. 

 It collects the blood from the veins and drives it up the gill-arches, from which it 

 flows out at the top into the dorsal aorta as arterial blood. Behind the air-bladder 

 is the kidney, formed of right and left halves intimately united, and connected by 

 folds of mesentery with the dorsal surface of the body-cavity are the ovaries in the- 

 female (the roe) and the testes (the milt or soft roe) of the male. 



As a general rule the eggs of fish are fertilised after they have been deposited 

 by the female, and it may be observed that the fewer the eggs are in number the 

 more anxiously are they looked after bygone or both parents. The various species 

 of catfish all appear to be attentive to the fry for some time. Some notions as 

 to the development from the egg may be gathered from fig. 8. While still within 

 the body of the mother, the egg measures about one-eighth of an inch in diameter ;. 

 it has two coats, the outer of which is penetrated by minute canals through, 

 which the necessary nourishment for the growth of the egg passes inwards. When- 

 the egg is laid, the space between the two coats increases in size and the two con- 

 stituents of the yolk) the formative yolk, which gives rise directly to the body of" 

 the embryo, and the large food-yolk which is utilized as food by the embryo) 

 formerlv evenly distributed, now tend to accumulate at opposite poles. The forma- 

 tive yolk with its contained nucleus begins to divide, the result being a disc of small 

 cells lying on the surface of the food -yolk. The cells gradually extend over the 

 whole of the egg, those at the formative pole arranging themselves into the three 

 layers of the embryo, which already during the second day assumes a fish-like- 

 form. It is from these three layers that all the organs of the fish are developed. 

 The embryo does not escape from its shell till the sixth day, when, although 

 only one-third of an inch in length, development has already advanced to a con- 

 siderable extent. Thus the heart is seen in front of the yolk-sac, from the vessels* 

 of which it collects the blood enriched by contact with the yolk, and propels it 

 by way of the gill-arches throughout the entire system. After all the food is, 

 36 (c.) 



