THE '"MOLLUSC A 189 



Then on its exit the water carries with it the waste pro- 

 ducts of digestion, and eggs, or embryos, as the case may be. 



Thus, except that larger particles can be taken in, a 

 lamellibranch mollusc lives and reproduces very much in 

 the same way as a sponge (the reproduction or extension by 

 budding in the latter, of course, excepted). The most 

 familiar example is the common oyster (Ostrea edulis). 



This is found on all our shores, but more especially where 

 the bottom is of such character as to afford points of 

 attachment for the embryo oysters when once they settle, 

 the conditions being clean, gritty substances, such as sharp 

 gravel or broken shell. (This will be best understood a 

 little farther on, when we get to the question of its re- 

 production.) 



The form of the oyster is too well known to require 

 description. It will just suffice to say that the flat valve 

 is the right side of the animal and the convex one the 

 left, for the oyster, like the large " Pecten," and one 

 other of our lamellibranchi (Pandora), is " inequivalved," 

 so that if an oyster is held hinge uppermost, and the flat 

 shell to our right, the " inhalant and exhalant apertures " 

 are towards us, the mouth on the further side. 



The reproductive glands, both male and female, are 

 represented in each individual, but do not mature at the 

 same time. Thus an oyster may function as female one 

 year and as male the following, or maintain the one function 

 for several successive years. 



When the eggs have been fertilised, in the manner we 

 have just observed, they, after the usual process of seg- 

 mentation of the yolk, give rise to little ciliated embryos, 

 termed Trochospheres. These are accommodated within 

 the parent, in a special pocket arrangement. If an oyster 

 is opened when it contains these trochospheres it appears 

 to the unaided eye as if partly filled with a milky fluid, 

 and is said by the oyster dealers to be " sick," 



