PISCES FISHES. 531 



gland provided with an excretory duct, wherein, in the female, ova are 

 secreted, and in the male a fecundating fluid is elaborated from the 

 blood. The eggs of the female when mature are expelled from the 

 nidus in which they were formed, and cast out into the surround- 

 ing water. The male, urged apparently rather by the necessity of 

 getting rid of a troublesome burden than by any other feeling, 

 ejects the seminal secretion in the same manner; and the fecun- 

 dating fluid, becoming diffused through the waves, vivifies the eggs 

 with which it is casually brought into contact. Such is the whole 

 process of reproduction in the osseous fishes. 



(577.) In the females of such fishes, the ovary, or roe as it is 

 generally called, consists of a wide membranous bag, ordinarily 

 divided into two lobes, but sometimes, as in the Perch, single 

 (fig. 227, q). This extensive organ, when distended with ova, 

 fills a large proportion of the abdominal cavity, and its lining 

 membrane is folded into broad festoons, wherein the ova are 

 formed, and lodged until sufficiently mature for expulsion. When 

 ripe, the eggs escape into the cavity of the ovary, and are expelled 

 in countless thousands into the surrounding element through the 

 orifice of the ovarian sac (Jig. 227, r), which is situated imme- 

 diately behind the anus (&), and in front of the urinary canal (Y). 



(578.) Generally, as has been already stated, the ova of fishes 

 are fecundated after their expulsion ; but there are a few instances, 

 as for example the Viviparous Blenny (Zoarcus viviparus) of our 

 own shores, in which the young are hatched in the ovary, and grow 

 to a considerable size before they are born : in such cases impreg- 

 nation must take place internally, and the males in these species 

 have, in fact, a nipple-like prolongation of the orifice of the duct, 

 through which the semen escapes, probably for the purpose of 

 introducing the seminal fluid into the interior of the ovary of the 

 females. Nevertheless, even in these the ovaria present the same 

 structure as in ordinary fishes ; the only difference being that their 

 eggs are retained until the embryo is far advanced in its develope- 

 ment, instead of being prematurely extruded. 



(579.) The testicle in the males of osseous fishes, generally 

 named " the milt," equals in bulk the ovary of the other sex, and 

 the quantity of the secretion furnished by it must be exceedingly 

 great. The entire organ is composed of slender and very delicate 

 convoluted caeca, in which the semen is elaborated. These tubes 

 towards the circumference of the testis all terminate in blind 

 extremities, but by their opposite ends they communicate with the 



