386 Causes and Course of Organic Evolution 



had a fresh-water origin in probably mid- or late-proterozoic 

 or archaean times, developed simple forms like Microhydra, 

 like the fixed stages of Limnocodium and Limnocnida or the 

 tentacular Hydra, and that from these in latest proterozoic 

 and early cambrian times marine types originated; while the 

 above fresh-water forms may have spread widely, and may 

 even, between the silurian and Jurassic epochs, have given rise 

 to many hydroid types now entirely lost. 



Included in the three great classes of the Coelenterata are 

 about 300 genera of Hydrozoa, a nearly equal number of An- 

 thozoa, and 25 genera of Ctenophora. From this one can 

 readily estimate how overpoweringly abundant the group is in 

 marine distribution at the present day. 



The Echinodermata seem very early to have assumed and 

 retained a marine existence, possibly from their early extensive 

 utilization of skeletal lime salts, so that from the time of the 

 Cambrian rock-deposits up to the present day their remains 

 or living representatives are abundant. But the fact that 

 they seem to be derived as a group from a primitive Dipleurula 

 larva with bilateral symmetry might carry them back to a 

 common ancestry with the fresh-water Enterocoela, while the 

 fact that some can live in brackish water, in spite of the great 

 majority being marine, indicates the possibility either of primi- 

 tive fresh- or brackish-water origin, or the capacity for adapta- 

 tion to such environment. But, before passing from them 

 in the present connection, it might be pertinent to ask why 

 they have not frequently and in different parts of the world 

 produced migrants from sea to fresh water, that might persist 

 for a time at least. Such seem to be as entirely unknowTi 

 as are Uke examples of the more highly modified genera of 

 Foraminifera, Radiolaria, and Porifera. 



The group is made up of 8 genera referable to the Crinoids, 

 58 genera to the Holothuroids, about 94 genera to the Aster- 

 oids, 98 to the Ophiuroids, and 220 to the Echinoids, or a 

 total of 478 genera; all which are marine, seldom indeed found 

 in brackish water. 



Regarding the Rotifera it has been said (130: 200) that 

 they "are of cosmopolitan distribution. Most of the species 



