Protoiheca of the Madreporaria. 7 



to how this can be requires unravelling. It may, for in- 

 stance, be the rim extended indefinitely and continuously as 

 a chalky film round a colony (e. g. Alveopira), or, again, it 

 may be discontinuous and represent the separate rims of an 

 aggregation of corals, each with its own cup, as in so many 

 Palaeozoic forms. In this case it depends upon the way in 

 which the corals are aggregated whether the rims are easy or 

 difficult to recognize. Add to these difficulties the fact that 

 apparently any part of the surface of a polyp may dis down 

 and secrete a calcareous film * which is purely adventitious 

 and has no morphological significance, and it is obvious that 

 until we had a key to its elucidation the epitheca could not 

 tail to be a source of bewilderment. 



Diagram 1 (PI. I.) shows the three earliest growth periods of 

 a primitive Madreporarian skeleton. All that we see is a deep 

 cup with three tabular floors. The process is explained in 

 diagram 2, in which we see three cups progressively modifying 

 their shapes. The lowest of these is the prototheca in the 

 strict sense of the word f, but it is advisable to apply the 

 term to all simple repetitions with free edges. Fig. 2 is so 

 far diagrammatic, inasmuch as with cups of this shape it is 

 impossible to say how far the rim of each cup extended before 

 the soft parts of the base of the polyp became detached from 

 the base of its prototheca. Cases, however, do occur in 

 which the change in the shape of the new thecas was rapid, 

 and for this and also for other reasons the rims of the separate 



* The formation of calcareous films somewhat irregularly over the skin 

 of corals is hardly to be wondered at. The prototheca was but the 

 primitive secretion of the basal portion of the polyp, forming a protective 

 cup into which the animal could retract the oral and exposed end of its 

 body. Above the ritn of this cup calcareous secretions were not usual, 

 otherwise they would have interfered with the process of retraction, but 

 the power of secreting them was not lost. Indeed, some forms actually 

 secreted lids, which, when the polyps retracted, closed down over the 

 prototheca (Calceola, Gomophyllum). A histological difference between 

 these secondary films and true epitheca may sometimes be noticed. The 

 former may be built up of separate plates, each of which starts round 

 some point of the skeleton and grows by concentric increments. 



t The prototheca is here drawn quite diagrammatic-ally. Figure 8, 

 after Lacaze-Uuthiers, is one of the best figures from life. My own 

 figures, already referred to, of a young Alveopora are of a prototheca 

 somewhat distorted. Theoretically we might expect a slight constriction 

 above the flattened sac, for as the soft larva settled down we might 

 expect its aboral end to flatten out somewhat wider than the neck 

 carrying the oral disk and tentacles. The base of the second prototheca 

 might easily be rounded or pointed, for it would hang down in the 

 hollow of the prototheca proper. The later development of convex tabulae 

 and vesicular dissepiments may have been due to the pulls of mesenterial 

 muscles. 



