xvn CLASSIFICATION OCCURRENCE 477 



The Gymnolaemata include three Sub-Orders : 



1. CvcLOSTOMATA. 1 Body-wall densely calcareous, the zooecia l>eing more 



or less tubular, usually with a circular orifice (Fig. 237). 



2. CHEitosTOMATA. 2 Body-wall of varying consistency. The orifice is 



closed, in the retracted state of the polypide, by a chitinous lip or 

 " operculum," which is more or less semicircular (Figs. 239, 241). 



3. CTENOSTOMATA.-"' Body-wall always soft. The cavity into which the 



tentacles are retracted is closed by a frill-like membrane, the edges 

 of whose folds have some resemblance to the teeth of a comb. This 

 membrane, the " collar," is seen in different conditions of protrusion 

 or retraction in Figs. 234, 238. The stomach may, in this group, 

 be preceded by a muscular gizzard (Fig. 238, C, <j). 



Occurrence. By far the larger number of the Polyzoa are 

 inhabitants of the sea. A recently published catalogue 4 of 

 marine Polyzoa includes nearly 1700 living species; and of 

 these, the great majority belong to the Gymnolaemata. This 

 group is further known to include an enormous number of fossil 

 forms. Not only do we find that in living Polyzoa the mem- 

 bers of a single Order largely outnumber the remainder of the 

 Polyzoa, but we may further notice that the Cheilostomata, one 

 of the sub-Orders of the dominant group, are at present largely 

 in excess of the whole of the rest of the Polyzoa taken together. 



Polyzoa may be collected with ease on almost any part of our 

 coasts. The fronds of the " sea-mat " (Flitstra foliacea) are 

 thrown up by the waves in thousands in places where the 

 bottom is shallow and sandy. The bases of the larger seaweeds 

 growing on rocks between tide-marks are nearly always thickly 

 covered with encrustations of Flustrclla hispida or of species of 

 Alcyonidiiim, in places where they are kept moist by being 

 covered with a sufficiently thick layer of other algae. liocks 

 which are protected from the sun may be coated with calcareous 

 Cheilostomes ; and these are also found, in company with branch- 

 ing Polyzoa of various kinds, on the bases of the Laminar ia 

 thrown up by gales or exposed at spring tides. The graceful 

 spirals of Biiyula turbinata (Fig. 233, A) may be found hanging 

 from the rocks at extreme low water ; while colonies of Srrupo- 

 t-cllaria, remarkable for their vibracula (see p. 484), are common 

 in many places between tide-marks. Certain species affect the 

 mouths of estuaries. 



1 xwXos, circle ; ffr6/j.a, mouth. - x'^ OJ > lip. ;i icrdt, KTtvfa, comb. 



4 Miss K. 0. Jelly, Synonymic Cat. Recent Marine Bryozoa, London, 1889. 



