234 



THE AUSTRALIAN MUSEUM MAGAZINE. 



Probably about sixty thousand dif- 

 ferent kinds of niollusca have been 

 discovered and named, but many re- 

 main yet unknown. About four 

 thousand are recorded from the par- 

 tially explored fauna of Australia. 



Naturalists have classified moUusca 

 into four main groups — the Cepha- 

 lopoda, the Gasteroj)oda, the Pelecypoda 

 and the Amphineura. 



last and largest of which it lives. An 

 example cut in half shows how a pipe 

 communicates with all the inner 

 chambers, by which the specific gravity 

 of the whole organism can be raised or 

 lowered The Post-horn shell (Spirula), 

 common on the ocean beaches near 

 Sydney, is similarly partitioned by septa, 

 but it is half internal, small and loosely 

 coiled. The shell possessed by the 



Pearly Nautilus shell, cut in half. As the animal moved forward to 



inhabit a larger room it built a fresh partition ; communication with each 



apartment is maintained by a central siphon. 



[Photo.— (^;. C. Chitton. 



CEPHALOPODA. 



Though largest in size, the Cephalo- 

 p::da are fewer in number of individuals 

 or species than the other orders. They 

 are exclusively marine, making a home 

 in every climate and on every coast 

 from the Equator to the Polar Circle, 

 and from the rock pool left by the 

 retreating tide to the abysses of the 

 ocean. This wide distribution through 

 geographical space is rivalled by that 

 through geological time, for the fossil 

 remains of these organisms occur in 

 every stratum from the youngest beds 

 to the ancient rocks of the Silurian 

 epoch. 



Like other mollusca, the Cephalopoda 

 are usually provided with a shell, 

 external in rare and archaic forms, but 

 usually internal. The Pearly Nautilus 

 has a shell of many chambers, in the 



Post-horn shell. Several joints broken open 

 to show the siphon. 



[C. Hediey, del. 



squid {Loligo) is a slender internal 

 spine or pen ; because this animal also 

 carries a vessel of sepia ink, it has been 

 called the " clerk of the sea." The 

 Cuttle-fish (Sepia) has a large, oblong, 

 white shell, or bone, which is also 

 internal ; this is commonly seen among 

 drift rubbish on the ocean beaches. 



