Evolution of Animals 391 



The Polyzoa form a group that may be regarded as pre- 

 eminently marine in the Hving species, of wliich there are 

 1700 included under 47 genera. The fresh-water species are 

 relatively few, and are included under 8 genera that are purely 

 fresh-water and 39 genera that are mainly marine but contain 

 fresh- water species. That the group is a very ancient one 

 is sho\^^l by the abundant remains of marine flustroid species 

 amongst the oldest palaeozoic rocks, and often side by side 

 with brachiopod or molluscan shells. These have been pre- 

 served on account of their tough membranaceous or more 

 often calcareous colonial shell or test covering. But it is 

 unquestionably true that all of these must have been preceded 

 by soft-bodied types. Now at the present day such are found 

 in di^dsions that contain, or consist of, fresh-water species. 



Thus Urnatella is a fresh-water genus of the subgroup Ento- 

 procta; Pahidicella, Boiverbankia, Alcyonidium, and Victor ella 

 are fresh- or brackish-water members of the Ctenostomata; 

 while the Phylactolsemata belonging to the Ectoprocta con- 

 tain only fresh-water species, though these seem to form the 

 most primitive group. 



But a persistently free-swimming habit in any of the Poly- 

 zoa, along with soft-bodied conditions, might indicate retention 

 of two ancestral and primitive states. This is still shown 

 to a hmited degree by the genera Cristatella, Lophopus, and 

 Pectinatella belonging to the fresh-water group Phylactolsemata. 



Structurally the Entoprocta have often been regarded as 

 the more primitive of the two main groups. But the retention 

 of the anus outside the lophophore circlet, the persistence of 

 a distinct body cavity that remains continuous with the zooids, 

 the retention of a muscular body-wall outside a mesodermic 

 layer, as well as the continuity of the body cavity with the 

 circular canal and thereby with the tentacular cavities, seem 

 all to point to the Phylactolsemata as probably most directly 

 descended from the most primitive division. On this view 

 the other groups and the genera of them — though now mainly 

 marine — may all have had a fresh-water ancestry, though in 

 extremely remote periods of the past. ^ 



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