STRUCTURE OF SEPIA. 339 



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-pericardial cavity. Through the 

 kidneys or nephridial sacs it is in indirect 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 modification on his fourth left arm. 

 The essential reproductive organs are unpaired, and lie 

 towards the apex of the visceral mass in the viscero-peri- 

 cardial cavity. 



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 com- 

 pacted into packets. These packets are found within the 

 next region — the spermatophore sac which opens to the 

 exterior to the left of the anus. Each spermatophore is like 

 a transparent worm. We may compare it to a little 

 glass-tube, closed at one end, drawn out and somewhat 

 twisted at the other ; within the tube at the closed end is 

 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. Now if the cement be soluble in water, and 

 the tiny machine be thrown into a basin, the spring will 

 expand violently as the cement is dissolved, 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 arrange- 

 ment, and its explosiveness in water. Even, indeed, on 

 a moist scalpel or on a slide, these strange but efficient 

 bombs will explode. The liberated spermatozoa are of the 

 usual type. 



