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) ; tl0D- 

 their cerebral ganglia lie above the commencement of the cesopha- 

 gus, 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 j it drives the blood into spaces between 

 the various organs of the body. Only the Cephalopods possess in- 

 ternal cartilages, but all are without an osseous endo-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 of two parts (valves), as in the The 

 Oyster, or may be single, as in the Limpet, or composed of a slieU - 

 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 invest the body, as in the Oyster, 

 lie within the folds of the mantle, as in the Sea-hares (Apli/sia), 



