POLYZOAN CORALS 



171 



{i.e. 30-200 fathoms). The colony is three or four inches in 

 height and is profusely branched more or less in one plane, 

 the branches freely anastomosing. The terminal branches 

 are usually flattened or compressed and terminate in blunt 

 points, or where they are about to bifurcate, in broad heart- 

 shaped expansions (Fig. 82). 



The main stem and the thicker branches are cylindrical, 

 and the calcareous substance of which they are composed 

 appears to be much harder and more compact than the 

 stems of other calcareous Polyzoa, but a close examination 

 of their structure in transverse section shows that they are 

 built up of concentric rings of zooecia with thickened walls. 

 At the surface the zooecia 

 are seen to be largely 

 submerged, but the orifice 

 of each one is raised on 

 a short conical projection 

 and this gives a rough 

 file -like texture to the 

 branches (Fig. 83). On 

 the flat terminal branches 

 the surface is smoother, 

 and as the walls are thin- 

 ner the complete outline 

 of the zooecia can be 

 more clearly seen. 



Porella is a very large genus, and among the many species 

 a great variety of form of growth is observed. Some 

 species are erect and ramified like Porella compressa, others 

 are flat and encrusting, forming large circular patches on 

 rocks and stones such as the common British shallow-water 

 species P. concinna. 



The genus Smittia is closely allied to Porella in the 

 structure of the zooecia and some species reach a considerable 

 size. S. landsborovii, for example, which occurs in British 

 seas, has a foliaceous variety which might be mistaken for a 

 small specimen of Lepraliajoliacea, but it can be distinguished 

 from the species by the presence of a small tooth-like pro- 

 jection on the lower lip of the orifice of the zooecium. 



Fig. 83. — Poi't'll.i compressa. Surface 

 view. On the right a part of a branch 

 showing the zooecia ; 8 dianis. On the left 

 a single zooecium ;■; 20 chams. 



