382 MOLLUSC A. 



Into the terminal portion of each nephridial sac, a little below its 

 aperture at the urinary papilla, there opens by a ciliated funnel another 

 sac, which is virtually the body cavity. It surrounds the heart and other 

 organs, and is often called the viscero-pericarclial cavity. Through the 

 kidneys or nephridial sacs it is in communication with the exterior. 



Reproductive System. 



The sexes are separate, but there is not much external 

 difference between them, though the males are usually 

 smaller, less rounded dorsally, and with slightly longer arms. 

 When mature the male is easily known by a strange modifi- 

 cation on his fifth left arm. The essential reproductive 

 organs are unpaired, and lie in the body cavity towards the 

 apex of the visceral mass. 



The testis an oval yellowish organ lies freely in a peritoneal sac 

 near the apex of the visceral mass. From this sac, the spermatozoa pass 

 along a closely twisted duct the vas deferens. This expands into a 

 twofold "seminal vesicle," and gives off two blind outgrowths, of which 

 one is called the "prostate." The physiological interest of these parts 

 is that within them the spermatozoa begin to be arranged in packets. 

 In this form they are found within the next region the spermatophore 

 sac which opens to the exterior to the left of the anus. Each spermato- 

 phore is like a transparent worm of complex structure. Think of a little 

 glass tube, closed at one end, drawn out and somewhat twisted at the 

 other ; see within the tube at the closed end a bag of dust attached to 

 and kept in its place by a sort of spiral spring ; this is prevented from 

 expanding by the fact that its upper end is fixed by cement in the mouth 

 of the tube. Suppose the cement be soluble in water, and that the tiny 

 machine be thrown into a basin, the spring will expand violently as the 

 cement is dissolved, and the bag of dust will be torn out and scattered. 

 Somewhat similar but more complex is the spermatophore with its 

 clear case, its contained bag of spermatozoa, its spring-like arrangement, 

 and its explosiveness in water. Even, indeed, on your scalpel, or on a 

 dry slide, these strange but efficient bombs will explode. The liberated 

 spermatozoa are of the usual sort. 



The ovary a large rounded white organ lies freely in a peritoneal 

 sac near the apex of the visceral mass. From this sac the eggs pass 

 along a short direct oviduct, which opens into the mantle cavity to the 

 left of the anus. Associated with the oviduct, and pouring viscid 

 secretion into it, are two large " nidamental glands," of foliated structure. 

 Close beside these are accessory glands, of a reddish or yellowish colour, 

 with a median and two lateral lobes ; while at the very end of the 

 oviduct are two more glands. All seem to contribute to the external 

 equipment of the egg. 



The spermatophores pass from the genital duct of the male to the fifth 

 left arm, which becomes covered with them and quaintly modified. This 

 is usual among cuttlefish, indeed in some, e.g.> Argonauta and Tr em- 

 octopus^ the modified arm with its load of spermatozoa is discharged 



