INTRODUCTION. 19 



Epitheca} — This structure is occasionally seen both in simple and in compound 

 corals ; it is the " foot-secretion " of Dana," and may either be closely applied to the wall 

 of the corallite or may simply cover the costal, leaving them more or less perfect in their 

 ornamentation. In some simple corals it covers the wall so closely as to resemble a 

 coating of varnish, in others its texture is rough and marked with concentric or encircling 

 ridges, and in a few instances it is marked by chevron-shaped lines. The epitheca may 

 be very thin or very dense, and it may simply cover the base or only reach a short 

 distance upwards from it ; or it may cover all the external surface as far as the calicular 

 margin. The dense epitheca of some MontlivalticB is accompanied by a great diminution 

 in the strength of the wall ; this is seen also in many Hugose corals. The epitheca of 

 compound corals is rarely ornamented, but is laminate and often readily destroyed. Its 

 preservation in fossils is comparatively rare, and it should therefore not be made of very 

 great classificatory value. 



The epitheca developes processes in certain species and only covers the base of others ; 

 it is porcellanous in some, as in Flabelluvi, and pellicular in others, as in Balanopliyllia. 

 It is membranous, striated, verrucose, marked by growth-rings, shining, rough and partial, 

 in diiferent species. 



It is a structure evidently formed after the development of the costse, and results fi'om 

 a tissue which is a continuation of that which determines the agglutination of the bases 

 and peduncles of certain corals to their supporting earth, stone or rock, or foreign 

 organism. 



III. — Anatomy of the Soft Tissues. 



The membranous surface which covers the calice, supports the tentacules, and is 

 perforated by the mouth, is called the Tentaculiferous Disc? 



The opening of the mouth is central, and is either circular or elliptical in outline ; it 

 is at the top of a truncated cone^ whose base is continuous Avith the disc and whose 

 height varies according to circumstances. The margin of the opening — the lip — is usually 

 marked by radiating ridges, is very prehensile, and can be moved in different directions. 

 The cone, whose upper extremity is the mouth, varies in its power of protrusion in 

 different species ; this is especially great when the tentacules are small and are only 

 arranged at the margin of the disc ; and, as a rule, when the tentacular development is 

 considerable the labial protrusion is slight. In some species, such as Heliastrcea cavernosa 

 and Litlioplyllia Qiibensis, there is a considerable space between the mouth and the 

 tentacules, and these last are feebly developed ; consequently the mouth can be so pro- 

 truded as to form a hollow between its cone and the base of the tentacules. 



1 Plate I, fig. 10. "- Plate lY, fig. C. 



3 Plate 11, figs. 4, 9, 11, 10, 13, 14, IG, 17. * Plate II, figs. 10, 11. 



