44 FIRST BOOK OF ZOOLOGY. 



separate, instead of being united. This figure represents the 

 tubes only partly extended. They can be thrown out to 

 twice the length represented in the figure. 



excurrent siphon. 



incurrent or branchial 

 siphon. 



FIG. 46. 



Another species quite similar to the above occurs on mud 

 flats, in company with the common clam. If this be col- 

 lected alive and placed in sea-water, the creature will extend 

 its siphons, which are long and separate, and bend them in 

 coils. 



43. The pupils have now learned, among other things, 

 a few features regarding the position which certain bivalves 

 occupy in their native haunts : the fresh-water mussel creep- 

 ing by means of its foot through the mud or sand in which 

 it lives partly buried ; the salt-water mussel, fastened to 

 some place by means of its byssus ; the soft-shelled clam, 

 lying buried at some depth in the mud, and extending its 

 siphons to conduct the pure sea-water to its gills, and food to 

 its mouth. 



Oysters differ considerably from the animals to which 

 they are related, and which have just been studied. Instead 

 of being free, they grow attached by one of their valves to 

 the rock or to one another ; clusters of a dozen or more in- 

 dividuals of different sizes are found growing, attached to one 

 another, and forming large masses. At any oyster-market 



