416 



PHYLUM MOLLUSC A. 



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 have 

 slightly longer arms. When mature, the male is easily 

 known by a strange modification 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 an automatically explosive 

 bomb ; within the transparent shell 

 there lies a bag of spermatozoa, and a 

 complex spring-like arrangement. Even 

 on the scalpel or slide these strange but 

 efficient bombs will explode. The 

 liberated spermatozoa are of the usual 

 type. 



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 

 4 'nidamental glands," of foliated struc- 

 ture. 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 modification of one of the arms 

 is usual among cuttlefish ; indeed, 

 in some, e.g. Argonauta and Trem- 

 octopus, the modified arm, with its 

 load of spermatozoa, is discharged 

 bodily into the mantle cavity of the 



FiG. 220. Male of Argo- 

 nauta (after Jatta), show- 

 ing " hectpcotylus " arm; 

 compare Fig. 9 of female. 



