URINOGENITAL ORGANS. 



209 



placed on the ventral side of the vertebral column between it 

 and the air-bladder. 



They have a great longitudinal extension, frequently reaching 

 from the head to the end of tlie body-cavity, or even in some cases 

 extending into the caudal region. Their front ends are enlarged 

 into the so-called head-kidneys. The head-kidney, as was 

 shown by Balfour, consists of lymphatic tissue which occupies 

 the place of the pronephros of the larva. There are two 

 longitudinal urinary ducts which unite 

 posteriorly to form the single ureter. 

 This structure, which frequently has a 

 bladder-like dilatation, passes ventral- 

 wards on one side of the air-bladder 

 to open externally behind the anus, or 

 into the rectum, into which the genera- 

 tive duct may also open (some Sym- 

 branchii, Plectognathi , Pediculafi), oi', in 

 the Pleuronectidae, on a papilla placed 

 asymmetrically on the coloured side of 

 the body. Nephrostoraata are never 

 present. 



The generative and urinary open- 

 ings, whether separate or united, fre- 

 quently open on a papilla which may be 

 of some length {BUnniidae, Gohiidae, 

 etc.). In RJwdeus the opening of the 

 oviduct is prolonged in the breeding 

 season into a tube, by means of which 

 the female deposits her ova in the 

 shells of living bivalves (Fig. 123). The 

 ovaries are usually double, rarely single, 

 saccular bodies the walls of which are continued into the short 

 median oviduct which opens between the anus and the urinary 

 opening, or with the latter. In some Teleosteans the ovaries are 

 separate from their ducts, and the ova are dehisced into the body- 

 cavity whence they escape by two funnels which join to form a 

 short tube which opens to the exterior usually between the anus 

 and the ureter {Salmonidae, Muraenidae, etc.). In viviparous 

 forms development takes place in the ovaries or in the oviduct. 

 The testes are paired saccular bodies, and are apparently always 

 z-ii. p 



121. — Kidneys of Salmo 

 fario (after Hyrtl). D ductus 

 Cuvieri ; R kidneys ; U 

 ureter ; XJr efferent duft of 

 bladder ; Vi bladder-like 

 dilation ; Vs subclaviaa vein. 



