THE ANATOMY OF THE MADREPORARIA. 23 



calcification in the theca and in the ectoccelic septa are found to have 

 run into a continuous dark line (Fig. 5), which at a yet lower level 

 is joined by those of the entocoelic septa. There are thus three 

 separate centres of active coral secretion at three different levels. 



From Fig. 5 it is also obvious that by far the greatest thickness 

 of the coral is laid down peripherally, i.e., by the calycoblasts of the 

 extrathecal part of the polyp. About six-sevenths of the thickness 

 of the theca is due to these calycoblasts, while the remaining seventh 

 is formed by those internal to the theca. 



ii. Anatomy. — In spite of the great length of the branch on 

 which it is borne, the polyp is often comparatively short, measuring 

 from 5 mm. to 20 mm. 



As will probably prove to be the case in all the Imperforata with 

 free calyces (cf. Cladocora (4), Caryophyllia (6), &c), the polyp is so 

 continued over the lip and outer side of the calyx as to form a 

 covering for its exterior surface to a varying distance (the " Eand- 

 platte" of von Heider). In Lophohelia this continuation may extend 

 for about 15 mm., or even more ; often it measures much less, and 

 in it the relations of the various body layers are such as have been 

 already described in the forms referred to above ; the part of the 

 coelenteron enclosed in this " Rand - platte " is divided up into 

 ectoccelic and entocoelic spaces nearly corresponding to those inside 

 the calyx by the peripheral lamellae, which were at a former time 

 continuous with the more central mesenteries, but which have been 

 mainly cut off from them by the gradual growth of the theca up- 

 wards, though the continuity is maintained above the lip (Fig. 6). 

 This explanation, due originally to Dr. von Koch, must undoubtedly 

 apply to this and many other adult forms. 



The general anatomical relations of the polyp, and its agreement 

 with forms already described, are shown in the diagrammatic segment 

 of a transverse section (Fig. 6). The " Rand-platte," the mouth disc, 

 tentacles, and stomatodamm are all in accordance with the normal 

 type. The coelenteron of the living polyp is, as usual, lined by 

 endoderm and mesogloja, apposed directly (except for scattered 

 calycoblasts) to the corallum. At the point, however, where the 

 living polyp ceases, its c<jelenteron is separated oil' from the cavity 

 in the coral which it previously occupied by a plug of decaying (?) 

 tissue, in which no cell-elements or organic structure are recognisable, 



