PHYLUM OHORDATA 



239 



l.c 



nerve passes through a slit in the other. In some cases the 

 olfactory bulbs spring directly from the prosencephalon, as in the 

 Trout ; in others they are borne oa long olfactory peduncles (or 

 olfactory tracts, Fig. 894, olf. p.), as in the Cod. In some Plecto- 

 gnaths the spinal cord undergoes a remarkable shortening: in a 

 Sun-fish 2|- metres in length and weigh- 

 ing a ton and a half, the cord is only 

 15 millimetres long, being actually 

 shorter than the brain. 



Urinogrenital Organs. — The kidney 

 (Fig. 865, kd) is formed from the meso- 

 nephros of the embryo, and usually 

 attains a great size ; the pronephros 

 usually atrophies. The ureter (ur.) is 

 the undivided pronephric duct : it unites 

 with its fellow of the opposite side before 

 opening either directly on to the ex- 

 terior or into a urinogenital sinus. A 

 urinary Madder is formed as a single 

 or double dilatation of the ureter. The 

 right and left kidneys undergo more or 

 less fusion, and their anterior ends are 

 usually converted into adenoid or lym- 

 phatic tissue (kd.'), so that, while re- 

 sembling the rest of the organ in ex- 

 ternal appearance, they do not discharge 

 a renal function. 



The male organs of Lepidosteus may 

 be taken as an example of those of 

 Ganoids. The testis (Fig, 896, ts.) is a 

 paired, lobulated organ, the secretion of 

 which is carried by a large number of 

 vasa efferentia (v. «/.) into a longitudinal 

 canal {I. c.) lying alongside the ureter 

 (ur.). From this canal tubes are given 

 off which communicate with the urinary 

 tubules of the kidney or open directly 

 into the ureter, so that the seminal 

 fluid has to traverse the latter in order 

 to reach the urinary bladder (bl.) and 

 make its escape by the common urino- 

 genital aperture ('lo.y. ap.). In Teleostei there are no vasa 

 efferentia, but the posterior end of the testis is directly con- 

 tinued into a duct (Fig. 865, v. d.) which unites with its fellow 

 of the opposite side and opens either into a urinogenital sinus, 

 as in the Trout, or, as in the Cod, directly on the exterior, 

 between the anus and the urinary aperture. In the Eels the 



u^.ap 



Fio. 896.— Male organs of Iiepi- 

 dosteus. hi. bladder ; I. c. 

 longitudinal canal ; ts, testis ; 

 u.g. ap. urinogenital aperture ; 

 ur. ureter ; v. ef. vasa efferentia. 

 (After Balfour and Parker.) 



