INVERTEBRATE ANIMALS OF VINEYARD SOUND, ETC. 299 



Cape Cod westward and southward. These shallow waters consist of 

 nearly pure sea-water, which has a relatively high temperature, especi- 

 ally in summer, for it is warmed up both by the direct heat of the sun, 

 acting on the shallow waters spread over broad surfaces of sand, and by 

 water coming directly from the Gulf Stream, and bringing not only its 

 heat, but also its peculiar pelagic animals. The temperature at the 

 surface in August was 66° to 72^ Fahrenheit. Owing to this influence 

 of the Gulf Stream these waters never become very cold in winter, for 

 some of the small, shallow harbors never freeze over. The greater part 

 of the animals inhabiting these bays and sounds are southern forms. 



The second assemblage is a very peculiar one, which inhabits the 

 estuaries, ponds, lagoons, harbors, and other similar places, where the 

 water is shallow and more or less brackish, and very warm in summer, 

 but cold in the winter. The third group inhabits the shores of the 

 outer islands and headlands and the bottoms in moderately deep water, 

 outside of the bays and sounds. These outer waters are comparatively 

 cold, even in summer, and are no doubt derived from an offshoot of the 

 arctic current, which drifts southward along our shores in deep water 

 and always has a tendency to crowd against and up its submarine 

 slopes, in which it is also aided in many cases by the tides. In August, 

 the temperature of the surface was 62^ to 65°, of the bottom 57^ to 62^ 

 Fahrenheit. The animals inhabiting these cold waters are mostly 

 northern in character and much like those of the coast of Maine and 

 Bay of Fundy. The surface waters in the bays and sounds, although 

 usually somewhat warmer in summer than those outside, differ less in 

 temperature than the bottom waters. Consequently we find less differ- 

 ence in the surface animals. We have therefore found it most conveni- 

 ent to group all the surface animals together, as a special division of 

 those inhabiting the bays and sounds. In each of the groups or assem- 

 blages we find that certain kinds are restricted to particular localities, 

 depending upon the character of the bottom or shore. Thus there will 

 be species, or even large groups of species, which inhabit only rocky 

 shores ; others which inhabit only sandy shores ,* others which dwell in 

 the muddy places ; and still others that prefer the clean gravelly bottoms 

 where the water is several fathoms deep, &c. 



I have found it desirable, therefore, in describing the character of the 

 marine life of this region, to group the animals according to the locali- 

 ties which they inhabit, adopting the three primary divisions given 

 above, but, for greater convenience of reference, placing all the parasitic 

 species together in one group. The subdivisions of these groups will 

 be given under each, in the succeeding pages. 



The primary groups will stand as follows : 



1. The fauna of the bays and sounds. 



2. The fauna of the estuaries and other brackish waters. 



3. The fauna of the cold waters of the ocean shores and outer banks 

 and channels. 



