64 THREE CRUISES OF THE " BLAKE." 



to about one hundred fathoms in depth. With them are often 

 mixed deep-water forms, which extend their range to shallow 

 water without however being 1 characteristic of it. 



o 



As in other groups, the limits of many species of mollusks 

 are more sharply defined on the side of cold than on that of 

 heat. The difference between 45 and 40 F. may absolutely 

 check the distribution of a species which would find no incon- 

 venience in a rise of temperature from 45 to 80. As has been 

 observed in fishes, this limit is probably connected with the tem- 

 perature necessary for development of the young, rather than 

 with the resisting powers of the adult. 



It would seem as if the conditions existing on the floor of the 

 deeper parts of the ocean offered attractions for only a limited 

 variety of forms. The bottom is generally composed of ex- 

 tremely fine impalpable mud, and in many portions of the abys- 

 sal area offers no stones or other prominences as points of 

 attachment for sedentary mollusks. It is not quite destitute of 

 such irregularities, however, and all are utilized by the abyssal 

 population. In the absence of stones, most unusual selections 

 are made. The chitinous tubes of hydroids and the irregular 

 leathery dwellings of tubicolous annelids are occupied, after their 

 original owners are dead or dispossessed, by diverse little lim- 

 pets. The long spines of the abyssal sea-urchins offer a welcome 

 perch for species of Cadulus, which, when they grow too large 

 to find a satisfactory foothold, secrete a shelly pedestal which 

 serves them for life. 



A bivalve, Modiolaria polita, related to the ordinary mus- 

 sel of northern seas, spins a sort of nest of stout byssal threads, 

 in which it is completely concealed, and which protects in its 

 meshes not only the young fry of the maker, but various little 

 commensal mollusks of all orders. Only a small number of 

 mollusks live as commensals. Species of Stylifer, a small gas- 

 teropod, live associated with star-fishes, sea-urchins, and other 

 echinoderms. Dr. Stimpson discovered another living within 

 an annelid ; and they are often found imbedded in branches of 

 corals, of which they have become a part as it were. 



Those mollusks which live on algse and other vegetable matters 

 are almost absolutely wanting in the depths of the sea, where 



