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 characteristic of it. 
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 algz and other vegetable matters 
are almost absolutely wanting in the depths of the sea, where 
