THE SHELL GALLEEY. 



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 marlced external division into segments (as in Worms) ; ^°°* 

 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 for 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 

 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 Gymbulia, or altogether absent, as in Polypus ; it 

 may cover and protect the body, as in the Oyster, lie within the 

 folds of the mantle, as in the Sea-hares {Aplysiidce) and the Slugs, 

 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 

 as in the Snail or the Slug, and so give rise to an air-chamber, 



B 



