240 



HIGH SCHOOL ZOOLOGY^ 



tentacles may be withdrawn for protection, and these cells 

 together with connecting: tubes constitute the " coenoecium." 

 It may be of very different character in different foims, being 

 sometimes gelatinous, but oftener horny or calcareous. The 

 colonies may reach a considerable size, but the individuals 

 i-arely exceed one or two lines in length. 



Most of the Polyzoa fall into two orders, which are nearly 

 co-extensive with the Fresh-water and Marine forms respec- 



Fig. 168. — Portion of a colony of Plumatella enlarged. 

 1 and 2, expanded indivdduals ; 3, 4, 5, individuals in various stages of retraction 

 Into the cells, a, the lophophore ; 6, cesophagus ; c, stomach ; d, intestine ; e, anus ; 

 /, statoblast, attached to base of stomach. 



tively. The former, which* are abundant in our ponds and 

 streams, have a horse-shoe shaped row of the ciliated tentacles 

 and a cover (epistome), which closes the mouth (Phylactolsemata); 

 the latter (Gymnolgemata) — of which Paludicella is the only 

 Fresh- water example — a circlet of tentacles and no epistome. 



Some idea of the form of the intestine and of the mechanism 

 of retraction into the cell may be formed from Fig. 1 68. The 

 nesrvous system is in the form of a ganglion between the aper- 

 tures of the intestine, and there are no special sense-organs. 



