224 MARINE ANIMALS 



and likewise with increase in the resistance offered by the water. This 

 resistance is the greater, the more water particles are displaced, and 

 the greater the sum of their paths described in such displacement; it 

 varies also with the internal friction or viscosity of the water, which 

 depends on temperature, salinity, and probably on pressure. 



The reduction in specific gravity may be made in many ways. 3 

 There may be economy in the use of hard skeletal or shell materials 

 such as lime and silica. Pelagic relatives of benthic animals with a 

 skeleton or shell are accordingly characterized by great reduction in 

 such hard parts. The foraminiferan, Orbulina universa, from surface 

 waters, has a very thin shell with walls from 1.28/x to 18/*, while speci- 

 mens from the bottom have walls up to 24/*, in thickness. The pelagic 

 Globigerina are distinguished by thin-walled shells from the single 

 benthic species, Globigerina pachyderma, whose name indicates its 

 character. Calcium carbonate is also reduced in the shells of Fora- 

 minifera by increase in the size of the pores and by enlargement of 

 the opening of the shell. 4 The shells of pelagic crustaceans are uncal- 

 cified or weakly calcified in comparison with those of their benthic 

 relatives, and have a higher fat content as well, as shown by analysis: 5 



The pelagic sea cucumber, Pelagothuria, lacks the lime bodies 

 which are invariably present in its relatives. Among pelagic snails the 

 heteropods and pteropods have delicate shells or none at all. The 

 shells of the pelagic lamellibranch Planktomya are uncalcified. 6 Simi- 

 larly among the cephalopods, the benthic Sepiidae have a calcified 

 internal shell, while the pelagic Loliginidae and Oigopsidae have the 

 shell narrow, delicate, horn-like, and uncalcified. Among pelagic fishes, 

 especially weak swimmers, the skeleton is weak, little calcified, or 

 reduced, as in Regalecus and Crystallogobius. Reduction of weight is 

 achieved by pelagic copepods by depositing their eggs singly instead 

 of carrying the egg sacks with them. 



The most widespread means of reducing specific gravity among 

 pelagic marine animals is the taking up of large amounts of water. 

 The absolute surplus of weight remains the same, but the relative 

 difference is reduced. This water is ordinarily incorporated in connec- 



