[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, Ulva, 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- 
ater, 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; many are com- 
mon in the estuaries but much more abundant in the pure waters of the 
sounds ; anda large number which are occasionally found in the brackish 
waters, especialiy where but little freshened, have their proper homes in 
the pure waters outside. 
Most of our food-fishes frequent the ponds ard 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 
