392 PHYLUM MOLLUSC A. 



phore is like an automatically explosive bomb ; within the trans* 

 parent 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 

 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 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 is usual among cuttlefish ; indeed, in some, e.g. Argonauta and 

 Tremoctopus, the modified arm, with its load of spermatozoa, is dis- 

 charged bodily into the mantle cavity of the female. There its discoverers 

 described it as a parasitic worm, " Hectocotylus. " The lost arm is 

 afterwards regenerated. In Sepza, however, the modified arm is not 

 discharged, but is simply thrust into the mantle cavity of the female. 

 The spermatophores probably enter the oviduct, and burst there. 



The eggs, when laid, are enclosed within separate black capsules 

 containing gelatinous stuff, but the stalks of the capsules are united, so 

 that a bunch of "sea-grapes" results. 



GENERAL NOTES ON MOLLUSCS. 



From the description of these three types a general idea 

 of the structure of Mollusca may be obtained, but it should 

 be noted (i) that all the three types are specialised; (2) 

 that two small classes, the Solenogastres and the Scaphopoda, 

 are unrepresented in the descriptions ; (3) that in the three 

 classes to which the types belong there is much diversity 

 of structure, this being especially true of the large and 

 heterogeneous class of Gasteropods. 



In surveying the structure of the whole group, it is con- 

 venient to begin with the most striking of the external 

 characters the absence or presence of a well-developed 

 head region. 



In the Lamellibranchs or Pelecypoda the head is absent, 

 and along with it the tentacles, the radula, and the 

 pharynx with all its associated structures. Elsewhere a head 

 region, usually furnished with tentacles and eyes, and con- 

 taining within it a pharynx and radula, is always present. 



