BIOLOGICAL SURVEY OF WOODS HOLE AND VICINITY. 171 



2. THE LOCAL FAUNA AS INFLUENCED BY THE CHARACTER OF THE BOTTOM. 



Of the three factors enumerated above, the first (character of bottom) is beyond / 

 doubt the most effective one in determining the distribution of organisms within the 

 limits under consideration by us. It is a mere truism that solid objects are necessary 

 for the attachment of whole groups of fixed organisms, e. g., hydroids, Bryozoa, ascidians, 

 barnacles, etc., as well as of many algae. The presence of stones or shells is therefore 

 essential to the existence of such forms. The absence of a suitable basis of support we 

 believe to account in the main for the comparative scarcity of hydroids in Buzzards 

 Bay. Soft mud doubtless interferes, likewise, with the respiratory currents of many 

 organisms, and these, too, would be better fitted to live in Vineyard Sound. Other 

 forms, on the contrary, require a muddy bottom in which to burrow. Thus, many of 

 our local annelids and certain bivalve mollusks are, for the most part, restricted to 

 Buzzards Bay. In some cases, as stated above, the relation between fauna and bottom 

 is less direct, as witness the small tube-dwelling worms of the genus Spirorbis, which 

 commonly adhere to various algae. 



Since, as we have seen, Vineyard Sound and Buzzards Bay are rather sharply 

 distinguished from each other by the presence or absence of mud on the one hand, and 

 of clean sand and gravel on the other, it is natural that the most obvious distinction in 

 distribution should be that between the predominantly Sound-dwelling species and the 

 predominantly Bay-dwelling species. By reference to the lists of species contained in 

 chapter 111 it will be found that 40 per cent of the more prevalent species dredged by 

 the Fish Hawk in Buzzards Bay do not appear in the list of the more prevalent species 

 dredged by the Fish Hawk in Vineyard Sound; while 35 per cent of the species con- 

 tained in the latter list do not appear among the former. Our distribution charts, 

 likewise, reveal the occurrence of many species which are restricted wholly or chiefly 

 to Vineyard Sound, and a considerable number of others which are restricted wholly 

 or chiefly to Buzzards Bay. 



Furthermore, within each of these major bodies of water, the local distribution of 

 many forms is very obviously determined by the presence of one or another variety of 

 bottom. Thus it happens that many species whose occurrence in Vineyard Sound is 

 general are found in Buzzards Bay only in the adlittoral zone, particularly along the 

 Elizabeth Islands. Here the mud is less prevalent, and the bottom approximates in 

 character much of that to be met with in Vineyard Sound. A type of distribution 

 which is almost the converse of the last is met with in the case of certain mud-dwelling 

 species, which are of general occurrence throughout the bottom of Buzzards Bay, but 

 which in Vineyard Sound are confined to a few definite areas where mud is known to be 

 present (e. g., Yoldia limatula, chart 135). Vineyard Sound is divisible, as has been 

 already stated, into an eastern half, in which the bottom is predominantly gravelly 

 and stony, and a western half, in which the bottom is mainly of sand (see chart 227). 

 Accordingly, many species, particularly among the attached forms, are lacking in the 

 western half of the Sound, except in the littoral and adlittoral zones; while certain 

 sand-dwelling species (e. g., the "lady crab," Ovalipes ocellatus, and among fishes the 

 rays and flounders) are especially prevalent in that very region. Such cases as these 

 are not always easy to distinguish from those to be discussed presently, in which temper- 

 ature determines which half of the Sound is inhabited by a given species. The lower 



