GENERAL NOTES ON MOLLUSCS. 387 



debated. In Nautilus, although the animal lives within 

 the shell, the mantle fold is for some distance reflected over 

 it ; in the other series of Cephalopods, this process has gone 

 further, and, where a shell is present, it is entirely enclosed 

 within the mantle fold, and is at the same time much reduced 

 in size. In the extinct Belemnites the internal shell was 

 straight and chambered, but almost concealed by secondary 

 deposits of lime, secreted by the walls of the shell sac. In 

 Sepia, according to one view, the central laminated region 

 of the " bone " represents the remains of the chambered 

 shell ; the remainder corresponds to the secondary deposits 

 of lime in the Belemnites. In Loligo, there is no deposit of 

 lime, an organic chitinous pen only being left. In Octopus, 

 there is no trace of shell at all. According to some, the 

 shell sac, in which the shell or pen of Cephalopods is 

 formed, is to be regarded as equivalent to the embryonic 

 shell sac plus a mantle pocket. According to Ray Lankester, 

 there is no relation between the secondary shell sac and the 

 typical primitive Molluscan shell gland. 



The sexes in Cephalopods are separate. The male 

 elements are made up into packets or spermatophores, 

 which usually pass out on to one of the " arms," more or 

 less modified for copulatory purposes. The eggs are large, 

 and often surrounded by capsules. 



The eggs are always furnished with a large amount of 

 yolk. In consequence, development is much modified as 

 compared with other Molluscs, trochosphere and veliger 

 stages not being recognisable. 



General Notes on Molluscs. 



The characters of the widely spread trochosphere 

 larva, and many of the features of the simple Amphi- 

 neura, suggest that Molluscs arose from some worm 

 type, but beyond this all is hypothesis. Some of the 

 Amphineura, both in the form of the body and in the 

 arrangement of bristles on its surface, recall Annelids, but 

 this can hardly be regarded as an evidence of affinity, for it 

 is extremely improbable that such typically unsegmented 

 animals can have arisen from segmented worms. It is 

 perhaps more likely that Molluscs arose from a Turbellarian- 



