LIFE IN THE DEEP SEA. 581 



out by phophorescent animals. At depths 2,000 fathoms and 

 upwards the temperature of the water is never many degrees 

 above the freezing point. 



The nature of the food of deep-sea animals has been a matter 

 of some considerable speculation.* Owing to the lack of sun- 

 light in the depths, there is an entire absence there of vegetable 

 life, such as could build up the necessary food of the animals 

 living there, and thus render the cycle of life in those regions 

 self-supporting and complete as it is on land and in the shallow 

 seas. 



Dr. Carpenter tells me he dredged living calcareous algse 

 (Corallinacece) in the Mediterranean Sea at a depth of 150 

 fathoms. As far as I observed, the " Challenger ' : dredgings 

 did not on any occasion yield algse from a depth so great. 

 The greatest depth from which seaweeds were dredged by us in 

 any quantity was, I believe, 30 fathoms. It is a curious fact 

 that a species of Halo2iliila, one of the Sea Grasses, which are 

 flowering-plants which have become modified to a marine exist- 

 ence, was obtained by us in abundance off Tonga Tabu from so 

 great depth as 18 fathoms. At this depth it was, when we 

 obtained it, in full flower. 



The only plants which extend their range to any great 

 depth are certain lowly organized parasitic Tlmllopliytes, which 

 infest corals and bore for themselves branching tubular cavities 

 in the hard skeletons of their hosts. These parasites have been 

 found by Prof. Martin Duncan in corals which have been dredged 

 from a depth of 1,095 fathoms.f These plants, nourished on the 

 tissues of their hosts, are able to thrive without the aid of sun- 

 light, just as do fungi in dark cellars and mines. 



In the absence of plants amongst them, the deep-sea animals 

 have to derive their food entirely from the debris of animals and 

 plants falling to the bottom from the waters above them. This 

 cUbris is no doubt mainly derived from the surface Pelagic flora 

 and fauna, but also to a large extent composed of refuse of 



* See K. Mobius, " Wo kommt denn die Nahrung von den Tiefsee- 

 thieren her." Z. f. Wiss., Zool. 21. Bd. Heft 2. 



t P. M. Duncan, F.K.S., &c, " On some Thallophytes parasitic within 

 recent Madreporaria." Proc. Eoy. Soc, 1876, p. 538. 



