224 Introduction to Anwial Morphology. 



Chaetopod worms have existed since the days of 

 the Cambrian formation (Arenicolites, Histioderma, 

 Helminthites), possibly since the Laurentian (Scoli- 

 thus ?). Tube dwellers are found in Silurian and 

 Carboniferous rocks. 



CHAPTER XXXII. 



CLASS 12. — BRYOZOA* [Ehrenberg). 



Colonies with tree-like, or membranous, fixed stocks 

 (polyzoaria) supporting" numerous personse in cellsf, 

 and composed of an outer chitinous or calcareous 

 ectocyst or cuticle, and an inner endocyst or dermis 

 (rarely absent as in Rhabdopleura). They are rooted 

 by a process of the ectocyst, and bear the cells for the 

 monoecious personse on one or two sides of the stock, 

 or radially around it. Each cup rarely communicates 

 with its neighbour (except in Ctenosomata, where also 

 they have serrated mouths). They may be joined one 

 to another, or united by coenenchyma, and the 

 branches joined by stolons. The ectocyst consists of 

 indistinct epithelial cells and often of calcified, vertical, 

 hollow fibres. The endocyst — soft, ciliated, and con- 

 tractile — lines each cell, and is reflected inwards at 



* This and the next are probably Epicoelous (p. 47) ; and should probably 

 form a separate province. This class is also named Polyzoa {Thompso7i). 



t CristateUa, which has no ectocyst, gUdes along snail-like. Some 

 Selenarise move by their enormous vibracula. Lophopus, which has a 

 gelatinous ectocyst, is also capable of locomotion. 



