MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 533 



undergone a similar modification, thougli not quite to the same degree; more of the cellular 

 character is retained than at the liase, but the passage into the broad ectoderm of the column is 

 abrupt. It is here that the epitheca is formed, and manifestly it is nothing more than the 

 upturned continuation of the basal plate. The epitheca and basal plate are covered only on their 

 inner surface by the polypal tissues, while the septal upgrowths from the l)asal disk are clothed 

 on both sides. The first two parts of the skeleton can therefore increase in thickness and extent 

 only on one face; but the septa are added to on both faces. The epitheca as j^et is unconnected 

 with the peripheral septal edges, but in older polyps it rests upon their free exposed margins. 



POSTLARVAL DEVELOPMENT OF SIDERASTR^A RADIANS. 



In both its free and incrusting condition Siderasirasa radiaru is a very abundant coral around 

 Jamaica, and fertile colonies have been obtained, and the development of the larvse and young 

 pol^'ps followed throughout a period of seventeen weeks. 



In the earliest extruded larvse the oral aperture is already established, and the interior is 

 nearly filled with a vacuolated, parenchymatous tissue, containing numbers of zooxanthellfe 

 uniformly distributed throughout. Four pairs of mesenteries are present; two lateral pairs are 

 complete, but the dorsal and ventral directives are yet free. In later larvsB the ventral 

 directives are inserted on the stomodseum, and the fifth and sixth pairs of mesenteries have 

 appeared. The dorsal mesenteries were complete by the time the hirvic settled, the Edwardslan 

 stage being thus reached, but mesenterial filaments were found only on the first and second bilateral 

 pairs of mesenteries. The ectoderm is crowded with zooxanthellre at the oral pole, and a few 

 occur over all the layer, but })ecome very sparse in the older larvfe. At the aboral jjole the 

 nerve layer undergoes a strong development, and nematocysts are more plentiful than elsewhere. 



Wide slits and .spaces, both intermesenterially and below the stomodjeum, began to appear 

 in the larvae shortlj' after extrusion, and represent the permanent gastro-coelomic cavity. Soon 

 the whole of the central part of the vacuolated tissue breaks down, and the middle of the cavity 

 is occupied by a mass of organic debris, among which are zooxanthella? and granules of various 

 kinds. Extrusions of such debris were bften observed from the free swimming larva?. Many 

 of the larvae became attached to pieces of glass, and the young polyps could thus be examined 

 under the microscope in their living condition as transparent objects, and the development of the 

 various organs and skeleton followed step by step. The full account of the postlarval development 

 will be published shortly, but the salient results may be here briefly sunmiarized. 



Most of the larva? were pear-shaped, the swollen extremity as a rule being the oral or 

 posterior end in swimming. On fixation many grouped themselves together, and thus from 

 primarily free and independent organisms young colonies were derived. Six pairs of mesentei'ies — 

 the Edwardsian members complete, and the fifth and sixth pairs incomplete — were present in 

 the newly settled larva. 



Tentacles. — Six equal tentacles, representing a primary cycle, appeared a few days after 

 fixation; but are exceptional among all corals whose development has yet been studied in that 

 they arise from the exocoelic chambers, not the entocoelic, as is usually the case. Two or three 

 weeks elapsed before the entocoelic cycle began to appear, when the members developed either 

 simultaneously or in a successive manner. They were situated central to the first cj'cle to arise, 

 and for a long period remained smaller than the others. The development of the two primary 

 cycles of tentacles was thus centripetal, the outer exotentacles appearing first and the inner 

 entotentacles next. 



The entotentacles of the adult Siderastrsea are bifurcated toward their extremity, and in 

 the course of their development in the larval polyps the two halves were found to appear 

 independently, and with a period of .several weeks intervening. The common peduncle was 

 developed later, and raised the two moieties above the disk. The exotentacles remained simple 

 throughout. 



The second cycle of mesenteries having appeared, another series of tentacles protruded 

 from the six additional exocoelic chambers, and with the primarj' exotentacles formed an outer 



