488 



PHYLUM MOLLUSCA 



marking the chambers tend to be lobed. There is often a single or 

 paired " Aptychus," perhaps of the nature of an operculum. Most 

 of the modern forms seem to be more active than their ancestors, and 

 their shells have degenerated. But the line of degeneration is still 

 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 farther, and, where a shell is 

 present, it is enclosed within the mantle fold, and is 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, and forming the " guard " or 

 rostrum. The conical chambered shell, with a siphuncle, is known 

 as the phragmacone. It is produced anteriorly into a gladius or pro- 

 ostracum. In the extinct Spirulirosira the shell was spiral and 

 mostly internal ; it had a guard. In Spirula the shell can be caught 

 siglit of in the young animal, but it becomes surrounded by the 

 secondary mantle folds that form the mantle-sac It is a spiral 

 chambered shell, with a ventral siphon. Its relation to the dorsal and 

 ventral surface of the animal is the opposite of that of the Nautilus. 

 The shell is inside the animal ; in Natitilus the animal is inside the 

 shell. It seems that Spirula is a swift swimmer at great depths ; 

 though the empty shells are often cast ashore, the creature itself is rarely 

 seen. In Sepia, the narrowed tip of the " bone " probably represents 

 the remains of the phragmacone ; the bulk of the " bone " probably 

 corresponds to the pro-ostracum in the Belemnites. Besides lime 

 there is chitin in the " Sepia-bone." 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, and no mantle-pocket, save a trace, in the very 

 yoimg animal. 



