THE ANATOMY OP THE MADREPORARIA. 7 



familiar, and requires here no systematic description. The 

 theca, which terminates the branches of the corallum, is solid, 

 as in all such Imperforata. The septa, which are exsert 

 above the lip of the theca, are both ectocoelic and entoccelic, 

 but are only irregularly arranged in orders. In a polyp with 

 forty-eight septa, for instance, of which twenty-four are ecto- 

 coelic, the remaining twenty-four entoccelic septa are probably 

 divisible into six primaries, six secondaries, and twelve ter- 

 tiaries ; but, as they all are approximately of the same length, 

 this division is founded more on analogy than on distinctive 

 differences. The total number of septa, which probably varies 

 with the age of the individual polyp, is not necessarily a multiple 

 of six or twelve. 



Transverse sections of the corallum show, as has been 

 recorded for other forms, e.g. Cladocora (4), Caryophyllia (6), 

 that a dark line, indicating its earliest formed part, runs down 

 the centre of each septum, and may be termed a " centre of 

 calcification." In addition to these lines, however, sections, 

 so made that they shall just cut the extreme lip of the actual 

 theca, exhibit other "centres of calcification" between the 

 enlarged ends of the septa, i.e. they lie in the theca itself 

 (fig. 4). In sections at a lower plane the centres of calcifica- 

 tion in the theca and in the exoccelic 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 entoccelic 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 thick- 

 ness of the coral is laid down peripherally, i. e. by the calico- 

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

 of the thickness of the theca is due to these calicoblasts, 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, mea- 

 suring from 5 mm. to 20 mm. 



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



