EXPLANATION OF PLATES. 



PLATE i. 



FIG. I. Larva immediately on extrusion, viewed by reflected light. The uniform covering of cilia is 

 indicated, and also the distinction between the outer ectoderm and the solid internal endoderm. The 

 darker color of the broader oral pole is due to the presence of Zooxanthellae in the ectoderm. The 

 narrower aboral pole is anterior in swimming; the mouth is not yet functional. 



FIG. 2. Abnormal larva with two oral poles and one aboral. 



FIG. 3. Larva shortly after extrusion. The larva is now more swollen and transparent, so that four 

 pairs of mesenterial lines are seen; the mouth is also functional. Two pairs of mesenteries reach the 

 stomodaeum, and two other pairs are free. 



FIG. 4. A second-day larva just before settling. Six pairs of mesenteries are now present, three 

 pairs of which reach the stomodaeum. 



FIG. 5. Three larvae settling close together by the narrow aboral pole. 



FIG. 6. A group of seven third-day larvas which have settled so close together that their walls partly 

 overlie. Four pairs of mesenteries now reach the stomodseum, and on three of the individuals the 

 rudiments of the six exocoelic tentacles have appeared. 



FIG. 7. A living polyp two or three days after settling, viewed by transmitted light. The dark 

 outermost rim represents the epitheca; the next lighter zone is the flat margin of the polypal wall into 

 which the polypal cavity does not extend. The six entosepta are opaque, but in such a view there is no 

 evidence of the basal plate. The six tentacles are exoccelic and alternate with the septa. The internal 

 Zooxanthellse have accumulated mainly along the sides of the mesenteries; the ectodermal Zooxanthella: 

 have disappeared except immediately around the mouth. Diameter of original, 1.5 mm. 



PLATE 2. 



FIG. 8. Living polyp with a dorso-lateral pair of exosepta and a rudimentary pair of median exosepta 

 in addition to the six entosepta. 



FIG. 9 Polyp with six entosepta and six exosepta, the latter still revealing their dorso-ventral order 

 of development by differences in magnitude. 



FIG. 10. A living polyp of about the same stage as fig. 9, fully expanded and viewed from the side 

 as a transparency. Of the mesenteries only the insertions are represented. 



FIG. II. Polyp with twelve fully expanded tentacles, six large outer exoccelic and six small inner 

 entoccelic. Fifth week. 



FIG. 12. Polyp showing the irregular manner in which peripheral additions are made to the primary 

 septa. The ventral directive tentacle is double. Diameter of original, 1.7 mm. 



PLATE 3. 



FIG. 13. Expanded disc showing the doubling of certain of the entotentacles and the asymmetrical 

 position of others. 



FIG. 14. Expanded polyp viewed from the side so as to display the manner of appearance of the 

 second-cycle mesenteries on the column wall. 



FIG. 15. Expanded disc resting on the septa and exhibiting the relationship of the second-cycle mesen- 

 teries to the new entosepta and exotentacles. An additional exotentacle has appeared within the middle 

 sextant on each side, and the dorso-lateral entotentacles have each a single stem bifurcated distally. 



FIG. 16. Expanded disc showing the manner of increase of the tentacles. An exotentacle protrudes 

 from each of the two exocceles in the dorso-lateral and median sextants, but as yet there is only one from the 

 ventro-lateral sextants. The tentacle from each dorso-lateral entoccele is already bifurcated as in the adult. 



FIG. 17. The same polyp as in fig. 16 at a later stage. On each side a second-cycle tentacle (II) has 

 grown out over the entoccele of the median second-cycle mesenteries, and the four lateral first-cycle ento- 

 tentacles (I) are bifurcated. Diameter of original, 2 mm. 



FIG. 18. A larval polyp of three months in which the disc and tentacles are indrawn and the disc is 

 almost covered by the overfolding column wall. The epitheca is shown around the margin. 



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