522 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



throughout the endoderm, and thus give rise to the faint brown color of the larva? as a whole. 

 Only rarely do they occur beyond the oral region of the ectoderm. 



From time to time the zooxanthellse are seen to be set free from the ectoderm cells, and 

 in the end they wholly disappear from the outer layer, the larva becoming uniformly colored. 

 Occasionally they persist for a short time within the perioral area after the larva has become fixed. 



The presence of numerous symbiotic zooxanthelhe within such a restricted region of the 

 larva during its interpolypal existence, and their disappearance shortly after the larva is set free, 

 are phenomena upon which no explanation has yet been offered. In no adult corals are 

 zooxanthelhe found within the ectodermal cells; it is wholly a larval condition. 



When the larva 1 are first extruded, the oral aperture is usually indeterminable, but a very 

 minute opening appears shortly after, often situated a little to one side of the actual apex, 

 and later the wall immediately around the mouth becomes slightly depressed. Extrusions of 

 zooxanthelhe, and what seem to be yolk granules in a mucus-like mass, have been observed to 

 take place from time to time through the newly formed mouth. This phenomenon commences 

 shortly after the larva? are set free, and continues for some time. Lacaze-Duthiers (1873) has 

 figured the ejection of waste material actually taking place in the larva? of Astroides, and it is 

 also found to be a common occurrence in Actinian larva?. It is manifest that this is the larva's 

 method of getting rid of the surplus zooxanthella?, yolk, and cell debris remaining after the 

 formation of the narrow endodermal layer from the original nearly solid internal tissue. Sec- 

 tions of late larva? are generally found to contain free zooxanthelhe, and what seem to be cells in 

 process of disintegration (tig. 112), while in early larva 1 the interior may be wholly filled with a 

 compact vacuolated tissue (PI. XXV). 



Some time after their extrusion, the larva 1 may enlarge a little, and begin to lose their 

 opacity, or this may not take place until they settle. Through the more transparent walls the 

 internal attachment of the mesenteries can be seen, and their number and course determined. 

 Usually three or four pairs of mesenteries are indicated at this stage, one or both of the lateral 

 pairs generally extending as dark, thickened bands farther down the polyp than the two axial 

 pairs (PI. XVII. fig. 125). 



In nearly all cases sections of the freshly extruded larva reveal an almost solid interior, into 

 which the very narrow stomodseum has pushed, as it were, its way; also three or four pairs of 

 mesenteries are more or less developed (Pis. XVIII, XXV). Afterwards the four pairs of 

 mesenteries seem to grow quickly, so that by the time of settling all may be united with the 

 stomodieum, two additional pairs — the fifth and sixth developmental pairs — having appeared in 

 the meantime; the latter, however, never unite with the stomodseum for a long time after 

 fixation. All the six pairs of protocnemes were already present in freshly extruded larva? of 

 Tsojphyllia dipsacea. Tentacular protuberances seem never to make their appearance before the 

 fixation of the larva, nor has any trace of skeletal matter been observed during the free swimming 

 stage. 



Within a day or two after extrusion the individual larva? settle by the forward aboral pole, 

 on any suitable surface which presents itself, and usually independently of one another. Should 

 the aboral extremity of the larva be narrow, it rapidly flattens after fixation; the larva as a 

 whole shortens greatly, swells laterally, and for the first time the differentiation into basal disk 

 and column is established. 



Fixation may take place in close proximity to the parent colony, and in such cases 

 probably directly after liberation. It is a common occurrence to find a few young polyps 

 adhering to the lower, dead surface of colonies of Manidna and Fa/via. Lacaze-Duthiers (1899) 

 has also shown that in the Caryophyllia obtained from Port Vendres "bouquets" of the coral 

 have been produced, evidently by the larva 1 fixing themselves on the exposed region of other 

 corallites, and, thus attached, growing to their full dimensions and giving a semblance of budding 

 or fissiparity to the usually simple coralla. Von Koch (1890) also describes "aggregated" 

 colonies in BalanqpkylUa, which could only have originated in the same manner. 



A still more remarkable instance of colony formation, from the union of individuals originally 

 free and distinct, occurred during the fixation of the larvae of Siderastraea radians. The larva? 

 settled in groups in such close proximity that when expanded the polypal Avails pressed against 



