340 THE LIVING ANIMALS OF THE WORLD 
FhoU by IV, SavilU^Ktnty b.Z.S. 
OCTOPODS 
^ blue-spotted IVest Australian species 
which is so richly supplied with muscular 
tissues as to constitute an effective creeping- 
base. As a locomotiv^e organ this muscular 
area is usually known as the “ foot.” 
Of living molluscs some 50,000 distinct 
species have been recorded. The great 
majority of these organisms are, as is well 
known, marine. A very considerable number, 
however, are inhabitants of fresh-water; while 
a yet smaller proportion, like the Slugs and 
Snails and their allies, are especially adapted 
for a terrestrial existence. Excepting two 
relatively small and inconspicuous groups, the 
great natural division or sub-kingdom of 
Molluscs is separated by systematic zoologists 
'uto three main section.s or classes. The 
particular modification of the locomotive 
organ, or foot, serves, on the one hand, to 
readily distinguish the first or most highly 
organised group from the second or central 
class ; while the third or lowest one is as 
clearly separated from the second and first by 
the character of the shell. The first and most 
highly developed section includes such species 
as the Octopus, the Cuttle-fish, the Squid, 
and the several varieties of Nautiluses ; to 
the second or central group are referred 
all the marine and terrestrial Slugs and 
Snails with their innumerable modifications; 
while the third and lowest group comprises 
all the double-shelled or bivalve forms, such 
as Oysters and Mussels. 
In the Octopus and its alliesthe creeping- 
base, or foot, is prolonged round its margin 
into eight or more arm-like extensions. The 
anterior of these in the earlier phases of their 
development grow round and enclose the 
mouth, which consequently opens upon the 
centre of the locomotive surface. The arms 
of the octopus and other forms are in most 
instances provided throughout their length 
with complex sucking-disks, wherewith their 
owners can seize and retain their prey or 
walk with ease and celerity over the smoothest 
perpendicular or overhanging rock-surfaces. 
The octopus, as the type of its class, possesses 
Phott by W. Savillt-Kent, F.Z.S.J IMilftrd-on-Sta 
A TASMANIAN SQUID, OR CUTTLE-FISH 
Giant species of squids ha-ve been recorded in ivhich :he shorter tentacles 
measured P 2 feet, and the two longer ones as much as 
JO or /f.0 feet in length 
