[461] INVERTEBTATE ANIMALS OF VINEYARD SOUND, ETC. 167 



dant eel-grass also impede the motion caused by the wind, so that these 

 bodies of water are comparatively quiet under ordinary circumstances. 

 The same causes allow the water to become highly heated during the 

 summer. It is evident that the heat and quietness of the waters are 

 unfavorable for the rapid absorbtion of oxygen from the air, while by 

 the rapid decay of the dead materials of the bottom large quantities of 

 carbonic acid and other gases must be evolved, which would in some 

 cases soon render the water fatal to all animal life, were it not for the 

 presence of the eel-grass, Viva, and other plants that flourish in such 

 waters, which, while absorbing the excess of carbonic acid, also help 

 to give the requisite amount of oxygen to the water. During storms 

 the mud of the bottom is quickly disturbed, causing the escape of nox 

 ious gases, and rendering the water turbid, while the eel-grass is torn 

 up in large quantities, thus adding to the decaying materials of the bot 

 tom and shores. Moreover, in case of rain-storms or spring- freshets, the 

 sudden addition of large volumes of fresh water often causes great 

 changes in the density and character of the water, sufficient to kill spe 

 cies not adapted to such varying and peculiar conditions. 



We accordingly find that although animal life is usually very abun 

 dant, the number of species that habitually live and prosper in these im 

 pure and decidedly brackish waters is comparatively small. But such 

 as do occur are usually found in great quantities, and are remarkable 

 for their hardiness and ability to live under widely varying conditions. 

 Many of them are strictly southern species, which do not extend much 

 farther north ; but there are some, like the long clam, muscle, &c., which 

 extend even to the Arctic Ocean and the coasts of Europe. 



Many of the estuaries and harbors, and some of the ponds, have a 

 much freer communication with the sea, and then the water is less 

 brackish and generally less impure in other respects, and the number 

 of species of animals becomes much greater. In other cases the water 

 is so little brackish that the fauna is nearly identical with that of the 

 outer bays. A few of these species are almost restricted to the brack 

 ish waters, but by far the greater number are able to live in pure sea- 

 water, and are accordingly also found in the bays and sounds. There 

 are various degrees of preference shown by the different species ; some 

 are very abundant in the brackish waters and very seldom found out 

 side ; some evidently prefer the estuaries but are also abundant in the 

 sounds; some flourish equally well in both situations 5 many are com 

 mon in the estuaries but much more abundant in the pure waters of the 

 sounds ; and a large number which are occasionally found in the brackish 

 waters, especially where but little freshened, have their proper homes in 

 the pure waters outside. 



Most of our food-fishes frequent the ponds and estuaries, either for 

 the sake of food or for the purpose of spawning, and many spend the 

 earlier part of their lives entirely in such waters. It is apparent, there 

 fore, that among the few species of invertebrate animals living in the 

 brackish waters, there are some that are of great importance as food for 



