1 86 Elementary Zoology. 



three-layered animals. But in many of those creatures, which 

 are better termed Coelentera, there is a third layer of cells, 

 derived from both ectoderm and endoderm, which is interpolated 

 between them in a jelly-like matrix, the supporting lamella. 

 So that, although the two-layered condition is the characteristic 

 one of the Coelentera, some of them are truly triploblastic. 

 But in none of them is the interpolated Mesoglsea excavated 

 by a coelom. The one central cavity is both enteron (gut) and 

 ccelom. In the coelomata these two cavities are separate. 



The Coelomata contain all the types described in this 

 volume, with the exception of the hydra. And we have 

 described representatives of many of the important divisions. 

 Leaving aside certain small groups whose relationships are 

 a matter of doubt, and would require, therefore, a more 

 elaborate discussion than space can be found for here, the 

 Coelomata may be grouped round the following types of 

 animals : — 



(i) Liver fluke (Platyhelminthes) ; (2) Thread-worms 

 (Nematoda); (3) Sea-urchin (Echinoderma) ; (4) Earth- 

 worm (Annulosa) ; (5) Lamp-shell (Brachiopoda) ; (6) Sea 

 mosses (Bryozoa); (7) Snail (Mollusca) ; (8) Cockroach, 

 Astacus (Arthropoda) ; (9) Frog (Chordata). 



It may be useful to sketch briefly the characters of all of 

 these groups. The Mollusca, Arthropoda, and Chordata will 

 be dealt with more elaborately. 



I. Platyhelminthes. 



This large group includes, not oijly the Trematoda, repre- 

 sented by the liver-fluke, but the Planaria (fresh-water, marine, 

 and terrestrial worms), and the Cestoda (the tape-worms). 



By many the Nemertine worms are included in the same 

 great division. These " worms " are more or less flattened, 

 sometimes elongated in shape. They are not definitely 

 segmented, like the Annulosa, though traces of segmentation 

 are occasionally apparent, as in the metameric arrangement of 

 certain of the internal organs in Gnnda segmentata. The coelom 



