MOLLUSC A— THE NEPHRIDIA 



221 



pericardium as a simple median tube. Anteriorly it is connected with the peri- 

 cardium by a funnel, and near the middle communicates with the exterior by means 

 of a lateral urinary duct (Fig. 19, p. 12). 



C. Scaphopoda (Fig. 165, \k 193). 



Dentalium has a pair of symmetrical kidneys, one on each side of the hind-gut. 

 Each nephridium consists of a sac provided with short diverticula. The two nei)lni- 

 dia are connected by a tube above the anus, and open into the mantle cavity by 

 two apertures at the sides of the anus. If, as maintained l)y all authorities, there 

 are no reno-pericardial apertures, the Scaphopoda would be the only group of Molluscs 

 in which these apertures are entirely absent. Apart from the symmetry of the 

 kidneys, a fact to be specially noted is that the genital products pass out of the 

 genital gland into the right kidney (either by the bursting of the wall between the 

 two organs or through an aperture), and only reach the exterior, i.v. the niantb' 

 cavity, through the right renal aperture. 



It must, further, be noted that near the anus on each side, between it and the 

 renal aperture, a pore, the water-pore, occurs, the function of which is still doubt- 

 ful. If these pores really lead into the blood lacunar system of the body, as was 

 formerly maintained, and is still held to be jiossible, this would be the only known 

 case of the direct imbibition of water into the blood. 



D. Lamellibranchia. 



is always paired and symmetrical, and lies 



The nephridium (organ of Bojanus 

 below the pericardium and in 

 front of the posterior adductor. 

 Each nephridium is tuliular or 

 sac-like, opening at one end 

 through a funnel into the peri- 

 cardium, and at the other into 

 the mantle cavity. This com- 

 munication of the kidney with 

 tlie mantle cavity always takes 

 place above the cerebrovisceral 

 connective. 



The lowest Lamellibrancliia 

 [Protobranchia, Nucula, Leda, 

 Solowviya) are distinguished 

 in two ways. (1) Each nephri- 

 dium is a simple tube, with a 

 free cavity not traversed by 

 trabecule or lamellae. This 

 tube consists of two portions 

 which unite posteriorly at an 

 angle ; the anterior end of one 

 of these portions enters the 

 pericardium through the renal 

 funnel, the other end opens 

 into the mantle cavity. (2) 

 The paired genital glands do 

 not open outward directly, but 

 enter the kidneys near their pericardial funnel — a fact which is very imjiortant in 



a la 



Fig. 184.— Transverse section through the body of Ano- 



donta, showing the pericardium, the heart, and tlie kidneys, 

 Cdnibined and diagrannnatised from figures by Griesbach. 

 Not all the parts represented occur on the .same section. 1, 

 Pericardium ; 2, ventricle ; 3, auricles ; 4, hind - g\U ; b, 

 venous sinus ; 6, reno-pericardial aperture (funnel) ; 7, renal 

 sac or cavity ; 8, vestibular cavity, which at 9 enters tlie 

 mantle cavity through the nephridial aperture ; 10, genital 

 aperture ; 11, base of the hint. 



