MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 521 



appeared singly, or only two or three at a time. Probably the larvae arc able to made their exit 

 through the tips of the tentacles, as well as through the mouth, though owing to the rapidity 

 with which the process takes place I have never Keen able to assure myself of this method, even 

 when, as in the ease of Siderastrsea, colonies have been watched for hours. Von Koch states 

 (1897) that the larvae <>l' Caryophyllia cyathus pass through the tipsof the tentacles. The various 

 polyps of a colony continue to give out larva? for several days, or even for a week or two. and 

 then the supply ceases, or for some time one or two individuals may appear at long - intervals. 



Development appears to proceed equally within most of the polyps in any colony. In the 

 fertile colonics of Faviaov Siderastrsea the majority of the polyps contained larvae all at the 

 same stage, and in Porites such was the case with most of the polyps within any restricted area. 



On first extrusion the larvae of corals are spheroidal, oval, pear-shaped, or elongated rod-like 

 bodies, varying from 1 to :i mm. in length, the outer surface uniformly ciliated throughout. The 

 various forms assumed by the different larvae of Famia fragum, extruded about the same time, 

 indicate the more usual shapes (PI. XIII, tigs. 96-100). Lacaze-Duthiers (1873) figures the larva 

 of Astroides calycvlaris as elongated and assuming a spiral form, von Koch (1897) that of (Jaryo- 

 l>jnjlliii cyathus as pear-shaped. The individual larva, however, often manifests the power of 

 retraction and of altering its shape, so as to he at different times oval, pear-shaped, spheroidal, 

 or flattened and cake-like. In the majority of cases, one end, usualh" the anterior during progres- 

 sion, is much broader than the other, though when the larva has been extruded for some time 

 these relations arc frequently reversed. Thus, the pear-shaped larva of Favia fragurn has at 

 first the broad end at the aboral forward pole, but later the broad end is oral or posterior 

 and the narrow end is aboral. A similar alteration of form has been noticed in Siderastrsea 

 rin//!//,.':, but most of the larva' of this species have a narrow aboral and a swollen oral pole, from 

 the beginning. Twin larva', with two oral extremities and one aboral, have been extruded by 

 polyps of the species just mentioned. No coral larva has yet been described in which the aboral 

 extremity bears the tuft of larger, less mobile cilia sometimes met with in Actinians. 



Coral larva? are able to swim about either immediately on extrusion or shortly after. For the 

 first few minutes the} - may remain motionless, either at the surface of the water or on the bottom 

 of the vessel, then, cilia having been formed, active movements of both rotation and translation 

 commence. Some gyrate throughout the depth of the water, coming to rest from time to time; 

 others remain nearer the surface or accumulate around the sides of the vessel. Within one to 

 three days a few of the more vigorous examples would become attached to the sides of the 

 glass vessel or other object, and remain thus for some time, then become active again, and 

 afterwards refix themselves. But the great majority seemed unable ever to settle, and continued 

 alive for days or weeks, without much motion, and apparently without undergoing further 

 development. If fixation did not take place during the first two or three days it was never found 

 to occur after, though some of Wilson's larvae of Manicina settled after swimming around for 

 three weeks. 



When first extruded, coral larva? are dense and opaque, and either colorless or slightly 

 brown; afterwards they may become slightly distended, and as a result the wall appears thinner and 

 more nearly transparent. Occasionally the larvae are set free in a distended form, when the}* are 

 more transparent from the beginning. The alteration from the opaque to the more transparent 

 condition is brought about by an important change in the internal endodermal tissue, described 

 below. Under the microscope the colorless or nearly colorless outer ectoderm can be distin- 

 guished from the inner endoderm, and in all species examined the latter bears zooxanthellae, 

 which give a yellowish color to the internal mass. 



The oral pole, whether narrow or broad, is usually darkly colored externally. Examination 

 of the living larvae under the microscope, and also by means of sections afterwards, reveals that 

 the color is due to the presence of numerous zooxanthellae or yellow cells toward the oral end. 

 Usually these occur within the ectoderm cells, but sometimes, as in Ixitjthyllin dipx/iau (PI. XXV. 

 fig. 165). they crowd the endoderm cells around the oral extremity, and the ectoderm contains 

 comparativeh* few. In both cases the zooxanthelhe arc densely aggregated toward the apex, 

 which in consequence is the darkest area. In general the alga? are sparingly distributed 



