MARINE MOLLUSCS 



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interesting to note that in these animals the rectum does not pass 

 through the heart, as it does in nearly all molluscs, and that a 

 pair of horny or calcareous 'styles' or 'pallets' project from the 

 place where the two siphonal tubes begin to diverge. 



Several species of Teredo are to be met with on our coasts, but 

 they are so similar in general structure that the above brief descrip- 

 tion applies almost equally well to all. 



Other boring molluscs frequent the British shores, but they 

 belong to quite a distinct family called the Gastroclicenida because 

 their shells gape widely on the ventral side. Their valves are 

 equal in size and very thin, the hinge has no teeth and the pallial 

 line is sinuated. The margins of the mantle lobes are thickened 



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FIG. 136. Gastrochana modiolina 

 I, Animal in shell : 2, shell ; 3, cell 



and united except where a small aperture is left for the protrusion 

 of the finger-like foot. The siphons are very long and retractile, 

 and the gills extend into the inhalent tube. These animals 

 burrow into mud, shells, or stone, often dwelling together in such 

 numbers that their galleries cross one another and form a most 

 intricate network, and the different species are to be found from 

 low-water mark to a depth of a hundred fathoms or more. 



The British species belong to two genera the typical genus 

 Gastrochcena, and the Saxicava or stone-borers. 



The former contains the Common Flask shell (G. modiolina) 

 which burrows into limestone and shells, in the latter case passing 

 generally through the shells into the ground below, and completing 

 its home by cementing together any fragments of hard material that 



