Il6 BULLETIN OF THE BUREAU OF FISHERIES. 



How the severe cold prevalent during the winter under consideration could have 

 resulted in the death of organisms dwelling in several (sometimes many) fathoms of 

 water is difficult to see. With animals so situated an actual freezing seems to be out 

 of question, and the temperature to which they were subjected on this occasion was 

 only a few degrees lower than that ordinarily endured by them in the winter. Further- 

 more, it must be pointed out that the peculiarities in the local distribution of Arbacia 

 correspond to known differences in summer temperatures, not winter temperatures. 

 As has been shown above (p. 50), it is likely that in winter all our waters attain 

 practically the same temperature at the coldest period of the year; and indeed it is the 

 shallower, more inclosed waters, such as those frequented by Arbacia, which are the 

 ones to respond most quickly to the winter cold. Further consideration will be given 

 to this subject in chapter v (p. 177). 



In addition to these illustrations, which have been discussed at length, we find 

 several other instances among this group of species whose distribution in local waters 

 is certainly related to temperature. Thus Asterias austera, Solaster endeca, and Gor- 

 gonocephalus agassizii, which reach their southern limit of distribution in this region, 

 have been taken by us only at Crab Ledge; while Asterias tenera, though recorded from 

 points as far south as New Jersey, is predominantly a northern form, and locally is 

 only known from outlying points such as Crab Ledge and Sankaty Head. Again the 

 brittle star Ophiopholis aculeata and the peculiar little holothurian Thyone unisemita, 

 the first of which, at least, is known to be a predominantly northern form, have only 

 been recorded by us from the western end of Vineyard Sound and from Crab Ledge — 

 a not unusual combination, as we have seen. 



Although it is a problem to what degree depth, as such, can be regarded as a factor in 

 determining the distribution of marine animals, we find of course many species which 

 appear to show marked preferences for the deeper or the shoaler waters of the region. 

 Among the echinoderms, it has already been pointed out that the sea urchin Strongylo- 

 centrotus occurs in Vineyard Sound chiefly at depths of 10 fathoms or more. The same 

 is true to a less extent of Asterias vulgaris. 11 Now both of these have already been 

 mentioned as northern forms, which are restricted in large measure to the colder waters 

 of the region. Their avoidance of the shoaler waters near land is probably dependent 

 upon their preference for lower temperatures. 



Some of our local holothurians have a converse type of distribution; i. e., they 

 show a decided preference for extremely shallow waters. To what degree this fact is 

 related to temperature, and to what degree it depends upon the character of the bottom, 

 in which they burrow, need not be considered here. One of this group, Thyone briareus, 

 was dredged by us several times but never far from land, and its more characteristic 

 habitat is probably in waters which are not accessible to the dredge at all. 



The following is a list of the echinoderms which were taken by us in the course of 

 the Survey dredging. The asterisk denotes as usual those species which were encountered 

 at 10 or more stations in Vineyard Sound and Buzzards Bay, and for which, consequently, 

 distribution charts have been plotted. 



u To a certain degree Henricia sanguinolenta is more prevalent in the deeper waters. Only 7 per cent of our records for this 

 species are from depths less than 5 fathoms, although 24 per cent of all our stations were at depths not exceeding that figure. 



