As the name ' Molluscs ' signifies, thti animals comprised 
under this sub-kingdom or phylum have soft bodies which, how- 
ever, in the majority of cases are protected by a calcareous 
shell. The shell may consist of a single piece, as in snails; or 
of two pieces, as in bivalves ; or of eight pieces, as in chitons. 
It is secreted by the mantle, a fold of skin which envelopes from 
above the greater part of the body. Below we find the foot, a 
thick muscular mass, which is highly characteristic of the 
Molluscs. It is used for locomotion and burrowing. A distinct 
head is present in all classes, except the bivalves. Besides the 
mantle (with its product, the shell), the foot and the head, a 
fourth organ is to be mentioned which is present in practically 
all Molluscs with the exception uf the bivalves and which is 
of great systematic importance. This is the radula, a thin 
ribbon-like chitinous membrance, beset with transverse and 
longitudinal rows of minute teeth. It rests upon the tongue, a 
kind of cushion on the floor of the mouth- Special muscles are 
attached to this tongue so that it can perform a licking move- 
ment and make its way through substances of great hardness, 
the teeth being siliceous. Seen under the microscope the 
radulae are beautiful objects, the teeth varying in size and 
shape, arrangement and numbers in the different species. 
The Molluscs may be divided into five classes, (I) Chitons 
or Amphbteura, (2) Snails or Gtistropoda, (3) Tooth-shells or 
Scaphopoda, (4) Bivalves or LiiniellibranchUita and (5I Cuttlefish 
or Cephahpodii. 
C H ITO N S (A^nphhieura). 
The Chitons are bilaterally symmetrical Molluscs, of an 
elliptical outline, somewhat of the shape of an egg cut length- 
ways into two halves. They are exclusively marine, and though 
found chiefly between tidal marks, sticking to rocks, a few have 
been obtained from great depths. They are readily distin- 
guished by the eight separate pieces of their shell which cover 
the curved back, lying behind and overlapping each other like 
the tiles of a rouf. This arrangement allows the animal to roll 
