BRYOZOA. 



07 



Phylum IV. MOLLUSCOIDEA. 



Class Bryozoa (Polyzoa).* 



The Bryozoa or Polyzoa are marine or fresh-water animals 

 almost always occurring in colonies, known as zoaria, which in- 

 crease by gemmation. Each zooid of the colony is enclosed in a 

 membranaceous or calcareous double-walled sac, the zooecium, into 

 which it can withdraw (Fig. 176^). The animal possesses a mouth, 



an alimentary canal, and an anal 

 opening,and, in addition to these, 

 a fringe of respiratory tentacles 

 — the lophophore (Fig. \j6b). 

 The colony is commonly attached 

 to foreign bodies, which it either 

 encrusts or from which it arises 

 as an independent frond. The 

 Fig. 176a. Membranipora pilosa\(xz- frond may be unilaminar, i e. 



cent). Two zooids, one expanded and with the cells opening on One 

 one withdrawn into the transparent zo- gide Qn] j t be encrusting 



oecium. Enlarged. ( After Farre. ) . 



or lined with a basal epitheca. 

 When the ends of such a unilaminar frond unite, a hollow 

 tube lined with epitheca is produced. Again the frond may be 

 bilaminat or bifoliate, with the epithecae of the two parts growing 

 together, forming a mesotheca. This often contains median tubuli. 

 Other forms are massy (hemispheric, globular, or discoidal in 

 shape) with gemmation on all sides ; ramose or branching and 

 dendroid. 



A network is often formed, when the branches grow together, 

 leaving fenestmles along the border of wh'ich are found the apertures 

 of the cells. The branches, instead of anastomosing, may be 

 straight and united at intervals by cross-bars or dissepiments 

 which commonly have no apertures (Fenestella, etc.). The branches 

 may be ornamented by nodules, spines or a longitudinal keel or 

 carina ; this latter may be sharp, expand, or even bear a com- 

 plicated superstructure. In some types (Archimedes) these fronds 



*Thin sections are required in the study of this class. 



