1006 



PISCES. 



sents itself under three principal types, each of 

 which will merit distinct consideration. 



The first is that observed in the Dermapte- 

 ryginous or Cyclostomatous Fishes, such as 

 the Myxine and Lamprey ; but it is not pe- 

 culiar to this group, seeing that the Eels and 

 perhaps other races have a similar organi- 

 zation. 



On opening the abdomen of one of these 

 Fishes, as, for example, the Lamprey, (Pe- 

 tromyzon tnarinus, Jig. 534) an exceedingly 



Fig. 534. 



' Female generative organs of the Lamprey 



(Petromyxon marimts). 



a, parietes of abdomen ; b, cavity of ditto ; 

 c, ovary ; e, external passage leading into abdo- 

 minal cavity, through which the ova are discharged; 

 g, kidney. 



extensive membranous expansion is found sus- 

 pended in loose folds, which is attached by a 

 kind of mesentery beneath the spinal column, 

 and extending along the whole length of the 

 abdominal cavity. Except in the breeding 



Fig. 535. 



One of the folds of the ovary of the Lamprey, 

 (Petromyzon marimu,) showing the enclosed ova. 



season, this membrane, of which a portion only 

 is represented in the figure, (Jig. 534, c,) is 

 thin and transparent, but at the same time 

 exhibits considerable vascularity. When the 

 breeding season approaches, innumerable gra- 

 nules begin to make their appearance, between 

 the two layers of which this expansion consists, 

 which in the female soon proclaim themselves to 

 be ova (Jig. 535), and, as they increase in size, 

 gradually distend the whole abdomen. On 

 opening the fish in this condition the abdominal 

 cavity appears to be completely filled with 

 innumerable ova beautifully arranged in rich 

 festoons, all of which are connected in front 

 of the spinal column. When the ova are 

 quite mature they are cast loose from the ovary 

 and escape from the ovarian membrane in 

 which they were formed, into the general ca- 

 vity of the abdomen, wherein they may be 

 found at this period floating quite loose prepa- 

 ratory to their expulsion. This is ultimately 

 effected through a simple but wide orifice (e) 

 situated immediately behind the anal aperture, 

 and causing a free communication to exist be- 

 tween the peritoneal cavity and the exterior of 

 the body, so that the ova easily pass out and 

 are ejected into the surrounding water. 



In the males of those Fishes which offer this 

 type of the generative system the appearance 

 of the reproductive organ is, while in a state of 

 inactivity, so exactly similar to that of the 

 female as to preclude the possibility of distin- 

 guishing the two from each other; but, as the 

 breeding season advances, the difference be- 

 comes apparent ; the festooned membrane, 

 which must in this case be called the test is, 

 secretes a kind of milt or seminal fluid, rich in 

 seminal animalcules, which in the same manner 

 as the ova of the female escapes into the peri- 

 toneal cavity and is expelled through a post- 

 anal orifice to be diffused through the sur- 

 rounding water, by the agency of which it is 

 applied to the previously deposited spawn of 

 the female, whose ova thus becoming vivified 

 are left to the mercy of circumstances to be 

 destroyed or hatched in due season. 



In the second form of the generative appa- 

 ratus which is common to almost all the true 

 osseous Fishes, a very different arrangement is 

 met with. The folds of the ovarian membrane, 

 instead of being loosely suspended in the abdo- 

 minal cavity, are now completely enclosed in 

 two capacious membranous capsules, situated 

 one on each side of the spine, and which when 

 distended with ova occupy a very large share of 

 the abdomen. On opening one of these cap- 

 sules, the ova which it contains are seen, how- 

 ever, to be developed between the two layers of 

 the proper ovary, exactly as in the case of the 

 Lamprey, and to be attached in broad festoons 

 to the interior of its walls, the essential diffe- 

 rence being that whereas in the preceding type 

 the eggs, when expelled from the ovary, escaped 

 into the peritoneal sac, they now are retained by 

 the capsular envelope of the ovary, whence they 

 are expelled through excretory canals provided 

 for the purpose. These, as they exist in the 

 Herring, are represented in the annexed figure 

 (fig. 536) ; from the posterior extremity of each 



