3382 



THE MOLLUSKS OR SHELLFISH 



a thick, often glossy periostracum, varying in color, the prevailing tints being 

 greenish olive, brownish yellow, brown, and black. Many are beautifully rayed 

 with green. The solidity of many of the species especially the North- American 

 forms is remarkable, although others are thin and fragile. Some are pearly 

 within, and others white, pinkish purple, salmon-color, yellow, or iridescent. 

 The shells are marked with adductor scars, and the pedal scars are also often 

 clearly visible, while the pallial line is uninterrupted by a posterior sinus. The 

 animal of Unio has the lobes of the mantle free, excepting posteriorly, where 

 they are connected, forming two orifices, the lower or branchial for the passage of 

 the water to the gills, and the upper for excretal purposes. The former is 

 fringed with several rows of papillae. The foot is large, thick, tongue shaped, 



PEARI, MUSSELS (Unio margaritifef) , SHOWING PEARI, WITHIN THE 



PEARLS (1-8). 



, AND DETACHED 



.and used as a creeping and burrowing organ. The sexes are united in the 

 European species, but distinct in the American. A remarkable feature in con- 

 nection with the Unionidce is the parasitic life of the early stages. The eggs 

 after being hatched between the gills of the parent, and having undergone the 

 first stages of development, subsequently develop into minute bivalves, each valve 

 having a hook-like process on the front edge, and are expelled from the brood 

 pouches. They at once attach themselves to some extraneous object, by means of 

 a byssal thread, and at this stage are known as Glochidia. They attach them- 

 selves, as soon as possible, to the gills and other parts of fishes, by means of the 



* 



valve hooks, and there complete their metamorphosis, finally quitting their host, 



sinking to the bottom, and assuming the parent form. Four species are found 



-in Britain, one of which (Unio margaritifer} is famous for the pearls it 



