POLYZOAN CORALS 159 



It is rarely possible to get the chance of seeing these 

 corals alive and expanded, but specimens which have been 

 preserved in spirit and examined in thin sections or in slices 

 cleared in oil usually show the essential characters quite 

 distinctly. 



The body wall of the polyp may be divided into two 

 regions, one of which is always thin and usually transparent 

 and is capable of being protruded with the tentacles, and 

 the other, which is thick and opaque and is connected with 

 the other polyps of the colony. The latter region forms a 

 receptacle called the " zooecium " into which the expansible 

 part of the polyp can be withdrawn telescopically for pro- 

 tection, and it is this part which secretes the calcareous 

 substance in the Polyzoa described in this chapter. The 

 outer wall of each zooecium is perforated b}' a large aper- 

 ture through which the polyp protrudes in expansion, and 

 this is called the " orifice," and may also be perforated 

 by a variety of other smaller apertures according to the 

 genus and species under observation. In many forms the 

 orifice is not flush with the surface of the zooecium, but 

 mounted on the end of a short spout-like projection which 

 may be called the " collar." 



There are no solitary calcareous Polyzoa, but every 

 species consists of a colony of many polyps whose zooecia, 

 firmly adherent to one another, build up the various kinds 

 of branching, net-like, or encrusting structures of the 

 Polyzoan corals. 



Most of the calcareous Polyzoa form little tufts of very 

 delicate branches or thin spreading plates on shells or 

 stones, and the term " corallines " is more generally applied 

 to them than " corals," but it is just as impossible to give 

 a scientific definition of the former as it is of the latter. All 

 that can be said is that when the word " coralline " is used 

 it has reference to something smaller or more delicate in 

 structure than what are commonly called " corals." 



The Poh'zoa are classified as follows : 



Sub-class I. Entoprocta. 



„ , , (Order I. Phvlactolaemata. 



2. hctoprocta - r- ' ^ ^ 



^ .,2. Gvmnolaemata. 



