EXPLANATION OF PLATES. 127 



PLATE 4. 



FIG. 19. Photograph of early corallum with six entosepta and six exosepta. The basal plate (proto- 

 theca) is fully formed, but somewhat irregular in outline; it is a little thicker and upturned towards the 

 margin. The six entosepta are nearly equally developed, while the alternating exosepta vary much in 

 size; the dorso-lateral pair are best developed and peripherally seem double, the middle pair are a little 

 smaller, while the ventro-lateral are yet rudimentary. Diameter, 1.6 mm. 



FIG. 20. A somewhat later corallum. The directive entosepta are irregular in form and differ 

 from one another, the ventral appearing as if bifurcated peripherally; the two dorso-lateral and right 

 middle exosepta also appear bifurcated ; the ventro-lateral exosepta are still much smaller than the other 

 exosepta. 



FIG. 21. A young preserved polyp photographed by transmitted light. In such a view the basal 

 plate is not apparent owing to its transparency. The septa are non-transparent and stand out as black 

 objects. Indications of the six pairs of radiating mesenteries are seen and also some of the tentacles, but 

 the details around the mouth are obscured by the mesenterial filaments and stomodceum. The dotted 

 surface of the polyp is due to the presence of internal Zooxanthellx. The septa are a little further 

 developed than in fig. 20, the exosepta still showing a decided dorso-ventrality. There is a marked 

 difference in form between the dorsal and ventral directive septa. Diameter, 1.7 mm. (In the process of 

 engraving the principal axis of the polyp has been turned counter-clockwise through an angle of about 

 20 degrees from the vertical.) 



FIG. 22. The same polyp as in fig. 21 photographed byan exposure of 20 minutes to polarized light 

 between crossed nicols. Only the calcareous crystals transmitted the variously colored light. The 

 mottled surface represents the basal plate which is not indicated in the previous figure. The crystalline 

 matter of the septa is so thick and irregularly arranged that no light was transmitted ; hence the structures 

 appear black. 



FIG. 23. A typical corallum at a later stage, growth having proceeded upon the septal plan of figs. 19 

 and 20. The polyp remained at this stage for some time, and a well developed wrinkled epitheca has 

 formed, its upper edge higher than the septa ; the latter are fused peripherally with the epitheca, but there 

 is no theca. The directive septa are somewhat bifurcated peripherally and also the dorso-lateral and 

 median exoseptal pairs. The dorso-lateral pair are almost fused centrally with the dorsal directive, the 

 middle exosepta are fused with the dorso-lateral entosepta on each side, and the ventro-lateral exosepta 

 with the ventro-lateral entosepta. The ventro-lateral exosepta are the smallest of the whole series. Sepa- 

 rate columellar nodules are present in the middle. For diagrammatic plan see p. 88. Diameter, 1.8 

 mm. (In the process of engraving the principal axis of the corallite has been turned clockwise through 

 an angle of about 20 degrees from the vertical.) 



FIG. 24. Another corallum at about the same stage as fig. 23, but showing variation in detail. 



PLATE 5. 



FIG. 25. A corallum in which the epitheca has gradually receded from the margin towards the 

 middle of the calice as the polyp shrunk in size. The older part of the epithecal deposit covers the 

 peripheral parts of the septa. The newest portion of the epitheca is the inner annulus. The septa are 

 at about the same stage as in fig. 23. 



FIG. 26. Corallum in which the polyp has twice diminished in size, somewhat suddenly. Three 

 concentric epithecze have been formed, the peripheral ends of the septa being partly exposed between each. 



FIG. 27. The septa are a little further advanced than in figs. 23-26. In the left dorso-lateral sextant 

 there are three distinct septa, a large median and two lateral; the median septum consists of a larger 

 central (exoseptum) and a small peripheral part (entoseptum). In the right middle sextant are also three 

 septa, the median of which consists of a central and a small peripheral part (cf. fig. 15). 



FIG. 28. This and the next corallum are the two oldest reared. Each contains three complete cycles 

 of septa, and some of the sextants show important stages in the passage from the second to the permanent 

 third cycle. In the polyps which formed the coralla the first and second cycles of mesenteries were fully 

 developed. The diagrammatic representation of the stage is given on p. 89, and emphasizes the manner 

 of fusion of the various septa with one another. The directive septa are stouter than any of the others. 

 The three septa within the ventro-lateral sextants are not so strongly developed as those within the median 

 and dorso-laternl sextants, and show very distinctly the twofold constitution of the median entoieptum 



