1 62 Descriptive Zoology. 



from the front, the anus, the opening of the oviduct, and 

 the opening of the ureter. In most of the higher fishes 

 these three openings are separate. 



Development of the Perch. The ovary is an elongated 

 body, occupying, when the eggs are mature, a large part of 

 the space in the body cavity. The outlet of the ovary is 

 the oviduct, whose external opening is just back of the 

 anus. The ovary shows that it is really a double organ by 

 its forked anterior end. In the male the two white sper- 

 maries unite in one sperm duct, which reaches the exterior 

 just behind the anal opening. The eggs are fertilized 

 after they have been laid. They are left without care on 

 the part of the parents. The eggs contain a store of nour- 

 ishment which is not yet completely absorbed when the 

 tiny fish begins to swim. The young fishes feed at first on 

 small crustaceans and other minute forms of life. Both 

 the eggs and the young fishes are eaten in great numbers 

 by many kinds of fishes and other voracious water animals. 



Scales. Scales are developments of the deeper skin or 

 dermis, serving for protection, or ornament, or both. The 

 scales usually overlap each other so much that only a small 

 part of each scale is exposed, and this part is covered by 

 the epidermis. The scales are usually of horny material 

 and not bony, except in a few fishes, such as ganoids. 



Kinds of Fish Tails. When the tail is completely sym- 

 metrical, inside and outside, it is called diphycercal. In 

 most fishes the tail, while externally symmetrical, is not so 

 within, the spinal column being turned up as it joins the 

 tail fin ; such a tail is homocercal. When the tail is unsym- 

 metrical, with the spinal column extending into the upper 

 lobe, the tail is said to be heterocercal. It is noteworthy 

 that the tails of nearly all young fishes are heterocercal. 



