Shells of Bivalves. 8 1 



interesting as one of the many evidences of the re- 

 lationship between worms and mulluscs. 



The shells secreted by molluscs consist of one, 

 two, or several valves, or pieces, and are very various 

 in shape, and often brightly-coloured. All molluscs 

 have a digestive canal, and sometimes a complex 

 arrangement of teeth. They have likewise a nervous 

 system, consisting of a ring around the fore-end of the 

 digestive canal, on which are formed ganglia over and 

 under the tube; besides this there are often other 

 nerve masses and organs of sense. There is a heart 

 of at least two cavities which propels the blood, but 

 there are few or no blood-vessels, the circulation being 

 chiefly carried on in the interspaces of the tissues. 

 There is rarely much of the body cavity to be found 

 free, with the exception of a small space around the 

 heart, which is called the pericardium, and from this 

 two short tubes called Organs of Bojanus pass out 

 representing the segmental organs of worms. Three 

 classes are included in this sub-kingdom. 



CLASS I. Lamellibranchiata. The representa- 

 tives of this group are oysters, mussels, cockles, &c. 

 These are easily recognised by their bivalve shells, 

 and by the two-lobed mantle under whose folds are the 

 gills or breathing organs arranged in layers or lamellae. 

 The fresh-water mussel, or the large Mya or collier, 

 easily found along our coasts buried in the sand, out 

 of which the tips of their long siphonal tubes project, 

 are good examples. The shell of one of these exhibits 

 to us a beak or umbo on each valve, and is marked on 

 its outer surface by numerous lines parallel to its mar- 

 gin ; the inner surface also differs in texture from the 

 outer, being whiter and often exhibiting a rnother-of- 

 G 



