40 8 FISHES 



CHAP. 



ducts or into the common sinus, and therefore both ducts 

 communicate with the exterior by a urinogenital orifice behind 

 the anus. Peritoneal funnels, similar to the functional ovi- 

 ducts of the female, are present in the males of the Chrondrostei 

 and of Amia. In Teleosts the terminal connexions of the ducts 

 tend to become less intimate. The archinephric ducts often dilate 

 into a urinary bladder either before or after their union, and 

 the common duct joins the united gonoducts to form a short 

 urinogenital sinus which opens externally, or the confluent 

 gonoducts have an independent genital orifice between the anus 

 and the urinary aperture. Not rarely the genital or the urino- 

 genital orifice is prolonged into a tubular papilla, which in the 

 male acts as an intromittent organ, or, as in the females 

 of the Cyprinoid Rhodeus amarus, the long oviducal tube 

 serves the purpose of an ovipositor. The males and females of 

 the Siluroid Plotosus have a remarkable vascular and glandular 

 arborescent appendage just behind the urinogenital papilla, the 

 use of which is unknown. 1 



The eggs of different Fishes 2 exhibit considerable diversity 

 in size and shape as well as in the nature of their external 

 coverings and their mode of deposition. 3 The size of the eggs 

 largely depends on the quantity of food-yolk stored up in their 

 substance for the nutrition of the embryo : hence the eggs of 

 Elasmobranchs, which resemble Fowls' eggs in the superabund- 

 ance of their yolk, are by far the largest. Teleostomi have 

 much smaller eggs. The. largest Teleostean ova are those which 

 are heavy and sink (demersal ova) ; the smallest, those which 

 are buoyant and float (pelagic ova). Of the former, the eggs of 

 Gymnarchus are about 1 mm. in diameter ; those of the Salmon 

 about 5 mm. ; and those of some species of Arius, 5 to 1 mm. 

 The eggs of the Wolf-Fish (Anarrhichas lupus) are about 6 mm. 

 Smaller demersal ova are those of the Lump-sucker (Cyclopterus) 

 and Heterotis, which are 2' 6 and 2 "5 mm. respectively. Pelagic 



1 Hirota, Journ. Coll. Sci. Imp. Univ. Japan, vii. 1895, p. 367. 



2 For the eggs of Cyclostomes see Chapter XVI. 



3 For a description of the eggs and breeding habits, and the larval develop- 

 ment and migrations of British Marine Fishes, see M'lntosh and Prince, Trans. 

 Roy. Soc. Edin. 1890; M'lntosh, Ann. Report Fishery Board for Scotland, 1892; 

 Cunningham, Marketable Marine Fishes of the British Islands, London, 1396 ; 

 M'lntosh and Masterman, Life- Histories of the British Marine Food -Fishes, 

 London, 1897 ; also numerous papers by Cunningham, Holt, Garstang, and Allen, 

 in the Joitrn. Marine Biol. Assoc. Plymouth, vols. i.-vi. 



