222 COMPARATIVE ANATOMY chap. 



connection witli the arrangement in tlie Solciiogastridm, the lower Prosohrmwhia {i.e. 

 the Diotocardia), and the Scaphopoda. 



In other Laraellibranchia also there is a relation between the genital glands and 

 the kidneys. In the Pedinidce and the AiwmiidcB the genital gland opens into the 

 kidney, but near its outer aperture. In Area, Ostnca, Cyclas, and Montaeuta, the 

 kidney and the genital gland open on each side into the base of a common depres- 

 sion (nrogenital cloaca) ; in all other bivalves the outer nephridial and genital aper- 

 tures are separate. 



The simple structure of the Protobranehiate kidney becomes complicated in 

 other Lamellibranchia in the following manner : — 



1. That portion of the renal tube ivhich opens outward forms an external cavity 

 (vestibular cavity, external sac) ; this cavity has no excretory epithelium ; it 

 encircles the outer side of the peiicardial portion of the kidney, the renal sac (Fig. 

 184). The latter alone is developed as an excretory organ. Folds or trabeoulfe, 

 covered with glandular epithelium, project inward from its walls, forming a paren- 

 chymatous or spongy structure. The renal sac is connected with the pericardium 

 by means of a nephridial funnel of varying length. 



2. The two renal sacs communicate freely in the median plane. The connecting 

 part is widest in the most specialised bivalves {Pholadaeea, Myacea, Anatinacea, 

 Septibranchia). 



In Anomia, where all the parts are asymmetrical, the two kidneys, which do 

 not communicate with one another, are also asymmetrical. 



Venous blood flows through the kidneys on its way to the gills. The afferent 

 renal vessels seem to have walls of their own, but the eflferent vessels appear to be 

 lacunar. Open communication between the blood vascular system and the kidneys 

 is nowhere found. 



E. Cephalopoda. 



(Cf. Figs. 185, 186, and the sections on the ccelom and the blood 

 vascular system, pp. 213 and 208). 



The Cephalopoda have two (Dibranehia) or four (Tctrabranehia) spacious sym- 

 metrical renal sacs, in the posterior and upper part of the visceral dome. These 

 communicate in the typical way at the one end with the ccelom, and at the other 

 with the exterior (mantle cavity). Only one of the two pairs of kidneys in Nautilus, 

 however, possesses ca'lomic funnels. 



The large veins returning from the body to the heart run along the anterior wall 

 of the urinary sac. These veins bulge out into the cavity of the sac to form the 

 venous appendages already mentioned. The epithelium of the urinary sac which 

 covers these appendages is no doubt the seat of the excretory function. The excretory 

 matter is discharged into the urinary sac (the wall of which is otherwise smooth), 

 and passes out thence through a ureter of varying length into the mantle cavity. 

 The renal aperture is found on the median side of the base of the gill, and in Nautilus, 

 the Oegopsidce, and Scpioteuthis among the Myopsidcc, it is simple and slit-like ; in the 

 other Myopsidce and in the Octopoda, however, it lies at the end of a renal papiUa 

 which projects freely into the mantle cavity. 



The two renal sacs in the Oetopoda are entirely distinct. Near the point where 

 the renal sac passes into the ureter lies the renal funnel, which corresponds with 

 the pericardial aperture of other Molluscs, and which here leads to the ccelomic 

 cavity, now reduced to the " water vascular system." 



In the Decapoda, the two renal sacs communicate with one another in the median 

 plane. In Sepia, there are two points of communication, one above and the other 

 below. The lower junction is bulged out to form a large sac,, which rises towards 



