THE PELAGIAL 225 



tive tissue, and thus produces the transparent jelly-like tissue, so 

 abundant among marine animals. It is known as the mesogloea in 

 coelenterates. Among medusae the amount of water may reach 99% 

 in Cyanea. 7 Siphonophores, heteropod and pteropod snails, Alciopidae 

 and Tomopteris among annelids, and chaetognath worms have 

 similar jelly-like tissue. Such jelly is widespread among the pelagic 

 cephalopods. Tremoctopus alberti resembles a small transparent ball of 

 jelly ; the transparent Alloposus mollis has so soft a body that it is cut by 

 the threads of a coarse net, the pieces passing through the meshes like 

 jelly through a sieve; the decapod Grimalditeuthis richardi is so trans- 

 parent that print may be read through its body. 8 The large vacuoles 

 in the parenchyma of pelagic turbellarians of the genus Haplodiscas 

 seem to be filled with jelly. The plankton fishes, Crystallogobius and 

 Aphia, and the eel-larvae, Leptocephalus, are also watery and trans- 

 parent. Among the Salpae and Pyrosomae, the cellulose mantle is 

 swollen with water. Invertebrate marine animals may take up water 

 from their surroundings without injury since their body fluids are 

 isotonic with sea water. 



More effective than the addition of sea water is the storage of 

 lighter materials, such as water of less salinity, fat, or even air. The 

 fluid in the vacuoles of the extracapsular body of the radiolarians, and 

 the protoplasm of Noctiluca, have a lower specific gravity than sea 

 water. 9 The ctenophore Beroe is said to have a fluid of low specific 

 gravity in the numerous vacuoles (of its cells) , which may be emptied 

 by muscular contraction at need. 



The accumulation of fat is widely distributed among pelagic ani- 

 mals, and this lowers the general specific gravity. This is not un- 

 common even among pelagic Protozoa. Radiolaria contain oil drops 

 in their intracapsular protoplasm, and the same is true of Noctiluca. 

 More extensive accumulation of fat is found in the pelagic crustaceans 

 such as cladocerans and copepods (see table on p. 224) . Numerous fat 

 bodies are present in the mantle of Planktomya. The selachians and 

 the cods (among bony fishes) store food material in the form of fat 

 in their livers. The eggs of many fishes are floated by the inclusion of 

 large oil drops, as in numerous Clupeidae, mackerels, and flatfishes. 

 Thick layers of fat enable the basking shark, Cetorhinus maximus, 

 and the sunfish, Mola mola, to sun themselves motionless on the 

 surface of the sea. Similar accumulations of fat among warm-blooded 

 marine animals, such as penguins, whales, and seals, serve also as 

 insulation. The amount of fat varies with the season, and in nonpolar 

 regions it is usually true that there is more fat in the warmer season 

 when the supporting power of the water is least. 



