464 



PHYLUM MOLLUSCA 



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 

 spermatophore 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 liber- 

 ated 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 " nida- 

 mental glands," of foliated structure. 



Fig. 262. — Male of Argo- 

 nauta (after Jatta), show- 

 ing " hectocotylus " arm ; 

 compare Fig. 9 of female. 



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. Argonaiita and Tremoctopus, the modified arm, with its 

 load of spermatozoa, is discharged bodily into the mantle cavity of 

 the female. There its discoverers described it as a parasitic worm, 



