ANATOMY OF THE LAMELL1BRANCHIATA. 303 



foreign rivers, and which perhaps have survived their immer- 

 sion in salt water during their voyage, by having kept their 

 valves constantly closed ; some species of Mytilus are known 

 to inhabit fresh water. It appears certain, that in those rivers 

 where the Uniones, Anodontae, and Cyclades abound, they 

 cease to be found where the water becomes salt. 



Having shown the fatal effect which would be produced by 

 the concentration of the sea water on the branchiae of its bi- 

 valve inhabitants, it is worth enquiry how, in those animals 

 which, on the retreat of the tides, are exposed to the desic- 

 cative action of the sun and air, this concentration is prevent- 

 ed. Those animals which have naked ciliated branchiae, have 

 the power of retracting them into sheaths, when they, like 

 many species of Doris, frequent the bare rocks ; or, if this 

 power of withdrawing them does not exist, as in other spe- 

 cies of Doris, the Tritonia, Eolida, &c, they take care to 

 cover themselves with the wet Algae, or to lurk in shady cre- 

 vices. The Patella, in hot days, sticks firmly to the rocks, 

 so as to prevent the escape of the confined moisture. The 

 Ascidiae frequent pools among the rocks, which are not drain- 

 ed at low water. The Actinia, Lobularice, &c, adhere to the 

 dripping under-surface of the cliffs, or frequent shady places. 

 The Polypi/era either reside in deep water, or find a habitat 

 where the sun does not reach them. Those Lamellibranchi- 

 ata which, like the common muscle, are exposed on the bare 

 rocks to the action of the sun and air, have the valves fitting 

 to each other most exactly, preventing all evaporation. — 

 When the valves are open at any part, the animal either in- 

 habits deep water, as many species of Pecten, or, when left 

 dry by the ebb of the tide, has the power of burrowing in the 

 mud or sand. The Gasteropoda also hide themselves from 

 the sun, although their branchiae are not much exposed. — 

 Walking along the sandy beach, we see numerous holes lead- 

 ing to the branchiae of bivalve and other animals, which, by 

 so boring, protect themselves from the effects of evaporation, 

 and obtain a supply of water, loaded with nutrient matter. 



The phenomenon called " animal phosphorescence" being, 

 perhaps, peculiar to eiliated animals, the author has endea- 

 voured to ascertain whether it may not be owing to the vi- 

 bration of these cilia. In an Annelide, perhaps the Nereis 

 noctiluca of authors, which presents this phenomenon very 

 beautifully, covering, in great numbers, the nets of the fisher- 

 men when they are taken up from ihe sea, he found that the 

 luminosity stopped when the action of the cilia ceased ; that 

 it was most vivid when they were most active ; and that the 

 tremulousness and unsteadiness, occasionally accompanying 



