THE SHELL GALLERY 
GENERAL NOTES ON MOLLUSCA. 
The Mollusca constitute one of the principal divisions of the 
Animal Kingdom, and include such animals as the Octopus, Cuttle- 
fish, Snail, Slug, "Whelk, Cockle, and Oyster. 
They may be characterized as soft, cold-blooded animals, without Defini- 
distinctly marked external division into segments (as in Worms) ; tion - 
their cerebral ganglia (the centre of the nervous system) lie above 
the commencement of the gullet, and are connected with the inferior 
ganglia by nerve-chords. Their heart consists of two or more 
chambers, and is situated on the dorsal side of the animal ; it drives 
the blood into spaces between the various organs of the body. Only 
the Cephalopods possess internal cartilages, but all are without a 
bony internal skeleton ; in the majority this is compensated by an 
external hardened shell which is formed (secreted) by the outer 
covering of the animal termed the mantle. The shell may consist The 
of two parts (valves), as in the Oyster, or may be single, as in shelL 
the Whelk and Limpet, or composed of a series of plates, as in 
the " Coat-of-mail " shells or Chitons : when well developed it is 
hardened by a rich deposit of carbonate of lime ; but it may be 
gelatinous, as in Cymbulia, or altogether absent, as in Octopus ; it 
may cover and protect the body, as in the Oyster, lie within the 
folds of the mantle, as in the Sea-hares (Aplysiidce), or it may be 
quite internal, as in the horny " pen " of the Squid. It may be 
elongated, as in the Elephant Tooth-shell (Dentalium), cup-shaped, 
as in the Limpet, or spirally coiled, as in the Snail. 
The mantle may form a free fold on either side of the body, as Descrip- 
in the Bivalves, or it may become largely attached to the body-wall, animal.** 16 
B 
