PEOPLING OF LAND AND SEA 155 



and frequent even in stagnant waters. The temperature and 

 the composition of the water varies incessantly. The ' ' Phytozoa 

 are hardly adaptable to this mobility. They have only a few 

 freshwater types, such as siliceous Sponges with straight 

 spicules {Spongilla, Parmida, etc.), a very small number of 

 Hydroid polyps or medusae {Hydra, Cordylophora, Limnocodium, 

 Limnocnida) , an Asiatic fluviatile Actina, Phylactolcematous 

 Bryozoa with gelatinous investment, and some Gymnolcemata 

 (A rachnidinm, Victor ella) . All these organisms seem to be recent 

 immigrants. It is remarkable that the freshwater Sponges and 

 Bryozoa should possess another method of reproduction besides 

 the sexual ; during critical times fragments of their body- 

 substance are enclosed in a protective envelope, which 

 enables them to withstand all the destructive agencies that 

 might menace them, and to escape the vicissitudes, so frequent 

 in fresh water. These fragments subsequently free themselves 

 from the envelope and evolve into new organisms as soon as 

 circumstances are again favourable. Examples are to be found 

 in gemmae or amphidisc capsules of Spongilla and the 

 statoblasts of the Bryozoa. 



The Crustaceans are represented only by Phyllopoda, 1 

 which go back to a very remote antiquity ; by a small number 

 of Cladocera, 2 Ostracoda, 3 Copepoda, 4 Isopoda, 5 Amphipoda, 6 

 Schizopoda, 7 and Decapoda. 8 These are very few in proportion 

 to the enormous number of Crustaceans extant. To them must 

 be added certain Arachnidas and Insects that have 

 become aquatic again after having led an aerial existence. With 

 certain rare exceptions, to which Charles Gravier has called 

 attention, the Annelid Worms present belong to the class 

 Oligochaeta, of which the Earthworm represents a giant 

 form, and to the class of the Leeches, which have only a 

 small number of marine forms, apparently imported by 

 migratory Fish. Among the Flat Worms only a small number 

 of Planarians and Nemerteans are encountered. The 



1 Estheridae, Apuscancriformis, Branchippus. 



2 Daphnidae, Polyphemidae. 



3 Cypridae. 



4 Cyclopes. 



5 Asellidse. 



6 Idothea. 



7 Mysis relicta. 



8 Caridina, Palcsmonetes, Alphceopsis, Niphargus, crayfish and other allied 

 genera, and some related crabs of the genus Telphusa. 



