MISSING GROUPS 



169 



there are no Actinozoa (sea anemones and corals) and no 

 Ascidia, or sea-squirts, otherwise abundant enough on our 

 sea-coast, though perhaps unfamiliar to many of us. 

 Other groups almost infinite in the number and variety of 

 their marine forms are barely represented by a few fresh- 

 water species ;. Gromia, and maybe one or two others, 

 stands for the great legion of Foraminifera. Of the 



FIG. 49. Hydra vulgaris, Pallas. Magnified 

 6 diameters. After Vosseler, from Lam- 

 pert, " Das Leben der Binnengewasser." 



sponges, the great majority are absent, there are no 

 calcareous sponges, no Hexactinellids, no Tetractinellids, 

 no horny sponges ; in the seas around our own shores 

 there are probably about 150 different species of this 

 group, in our rivers and lakes not more than half a dozen. 

 The same deficiency is manifested by the Hydrozoa ; 

 apart from missing families, there is the same contrast in 



