480 



POLYZOA 



viduals grow out at intervals from a cylindrical stem or " stolon 

 (st), which is not composed of zooecia. 



The Cyclostomata may assume an erect or encrusting habit. 

 Their zooecia are always more or less cylindrical ; the upper ends 

 being often completely free, although in many cases the whole 

 zooecium is closely adnate to its neighbours. In the breeding 

 season the forms which belong to this group are provided with 

 curious " ovicells," which contain the embryos. ' These may 



Fig. 238. Bowerbankia 

 pustulosa Ell. and Sol., 

 Plymouth. A, Fragment 

 of a colony, natural size, 

 showing the branching 

 stem, bearing tufts of 

 zooecia : B, one of these 

 tufts, with the growing 

 apex of the stem(s), x 27; 



b, young zooecia (buds) ; 



c, the "collar" charac- 

 teristic of Ctenostomata ; 

 t, tentacles : C, a single 

 zooecium, with expanded 

 tentacles, more highly 

 magnified ; a, anus ; c, 

 collar ; g, gizzard ; i, in- 

 testine ; o, oesophagus ; 

 s, stomach. 



either be pear-shaped swellings on the branches 



(Crisia, Fig. 



237), or they may form inflations of the surface, between the 

 zooecia. The mature ovicell is provided with one or more open- 

 ings, through which the larvae escape. 



The Ctenostomata rarely have even the slightest trace of calca- 

 reous matter. Alcyonidium and its allies form soft encrustations, 

 or may even grow into erect masses six inches or more in height 

 (A. gelatinosum). In this type the zooecia are often so closely 

 united that it may be difficult or impossible to make out their 

 limits in the living colony. Many of the dendritic or branching 



