402 FISHES CHAP. 



Salmonidae/ where they are as singularly variable in different 

 species and individuals as in the Elasmobranch Scylliidae. The 

 use of abdominal pores is not certainly known, unless the coelom of 

 those Fishes which possess them continues to retain some measure 

 of its primitive excretory function, and the pores act as excretory 

 ducts. That the nephrostomes are excretory organs has been 

 shown by experiment, and it is worthy of note that there exists a 

 reciprocal relation between these structures and abdominal pores, 

 to the extent that while there are a few Fishes (e.g. certain 

 Elasmobranchs and Amia) in which both coexist, there are 

 many others in which the presence of nephrostomes is correlated 

 with the absence of pores and vice versd. 



The male and female gonads, testes and ovaries, are derived 

 from the coelomic epithelium near the inner or median aspect of 

 the nephrotomes (Fig. 229, B). Here the epithelium remains 

 columnar, and soon projects into the ventral coelom as a con- 

 tinuous longitudinal ridge. It is probable that at first the modified 

 epithelium is segmented as a series of " gonotomes," but if so, the 

 latter must soon coalesce into a continuous ridge. Some of the 

 epithelial cells enlarge to form the primitive sex-cells. In the 

 development of an ovary, portions of the epithelium sink inwards, 

 carrying with them the primitive ova. Certain of the cells form 

 the epithelial walls of a number of ovisacs, each of which encloses 

 an ovum. As the ovisacs increase in number and size the 

 germinal ridges project more and more into the coelom until, 

 as ripe ovaries, they l^ecome suspended from its dorsal wall by 

 a double peritoneal fold, the " mesovarium " (Fig. 156). The testes 

 develop in a similar fashion except that the primitive sex-cells, 

 which later give rise to spermatozoa, form the lining of a number 

 of simple or ampulla-like tubules, the seminiferous tubules, and 

 the suspensory fold is termed the " mesorchium." 



The Cyclostomes have gonads in the shape of unpaired 

 organs extending nearly the whole length of the coelom, but 

 in all Fishes the organs are primarily paired, although by fusion, 

 or by the absorption of one gonad, the ovaries or the testes 

 sometimes appear as if single. The ovaries may either be naked, 

 as in Elasmobranchs, Dipnoi, Crossopterygii, and Choudrostei, 

 and in Amia amongst the Holostei ; or, as in Zepidosteus and 

 most Teleosts, they become enclosed in coelomic sacs. The 



^ Max Weber, Morph. Jahrh. xii. 1886, p. 336. 



