158 BULLETIN OF THE BUREAU OF FISHERIES. 



Far in excess of any of these figures is that expressing the number of species taken 

 at the seven Crab Ledge stations. This is 3.3 per dredge haul. Certain localities in 

 Vineyard Sound, likewise, notably the area between the Middle Ground and the shores 

 of Marthas Vineyard were especially rich in tunicates. For example, five species each 

 were taken at stations 63 and 7525, while six species were taken at station 7751. 



As in many previous cases which have been discussed by us, we believe that the 

 well-known difference between the bottoms of Buzzards Bay and Vineyard Sound is 

 chiefly responsible for this difference in the wealth of their ascidian faunas. This belief 

 is strengthened by a consideration of the average number of species per dredge haul 

 taken upon the three principal types of bottom which have been distinguished by us. 

 The figures, according to this basis of classification, are as follows: Mud, 0.4; sand, 0.9; 

 stones or gravel, 1.9. Moreover, as in many previous cases, some of the species which 

 are absent elsewhere in the Bay have been taken near shore, where the mud of the central 

 region largely gives place to sand, gravel, and stones. Such in particular are Styela 

 partita and Amaroucium pellucidum constellatum. 



As is well known, ascidians are dependent upon ciliary currents for the food and 

 oxygen brought to them in the water. It is thus natural that bottoms of soft mud should 

 not commonly offer them a congenial habitat, even though a suitable basis for attach- 

 ment should be present. The occurrence of stones, shells, and algae, or other suitable 

 bases of support is likewise an important factor in determining the distribution of most 

 species, as is evident from a comparison of the abundance of ascidian life upon bottoms 

 of stones and gravel with that upon bottoms of sand. Herein, also, probably lies the 

 explanation of the scarcity of bottom-dwelling tunicates in the western half of Vineyard 

 Sound. 



Of the seven species b for which separate distribution charts have been plotted, all 

 agree in being either wholly lacking in the western half of Vineyard Sound, or, if present 

 there at all, in being confined to the inshore (adlittoral) stations. As has been already 

 pointed out, this western area of Vineyard Sound (barring the inshore region) is charac- 

 terized by the presence of sand, and by the comparative absence of stones and gravel. 

 In the case of Styela partita, Molgula manhattensis , and Perophora viridis, it is possible 

 that distribution is in some measure determined by that of certain algae, since these 

 species are very frequently attached to the latter. An inspection of the distribution 

 charts for the algae, however, shows few species, if any, whose distribution would satis- 

 factorily account for that of the ascidians named. 



On a number of previous occasions, we have shown the likelihood that temperature 

 has been the factor chiefly concerned in excluding certain species from the western end 

 of Vineyard Sound. Various predominantly southern species seem unable to thrive in 

 the colder waters of the region, just as certain northern forms seem unable to thrive 

 elsewhere. Now an inspection of the table below, giving the ranges of our commoner 

 species of ascidians, shows that none of those listed are predominantly northward ranging 

 forms, while four, on the other hand, are predominantly southward ranging forms, some 

 of which, indeed, reach their northern limit in Cape Cod. Despite these facts, it seems 

 to us unlikely that temperature has been the factor chiefly concerned in determining the 



o Exception must be made in the case of those species occurring in deep-sea oozes, many of which are stalked. (Herdman). 

 b Two of these are not now regarded by Dr. Van Name as being specifically distinct, but for reasons stated above (p. 155, 

 footnote) their distributions have been plotted separately. 



