MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 533 



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

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

 abrupt. It is here thai the epitheca is formed, and manifestly il is nothing more than the 

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

 inner surface by the polypal tissues, while the septal upgrowths from the basal disk are clothed 

 on both sides. The first two pari- of the skeleton can therefore increase iii thickness and extent 

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

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



POSTLARVAL DEVELOPMENT OP SIDKKASTR.KA RADIANS. 



In both its free and incrusting condition Siderastraea radians is a very abundant coral around 

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

 polyps followed throughout a period of seventeen weeks. 



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

 nearly tilled with a vacuolated, parenchymatous tissue, containing numbers of zooxarithellae 

 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 larvse 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 larvae settled, the Edwardsian 

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

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

 occur over all the layer, but become very sparse in the older larvae. At the aboral pole the 

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



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

 in the larvae shortly after extrusion, and represent the permanent gastro-ccelomic cavity. Soon 

 the whole of the central part of the vacuolated tissue breaks down, and the middle of tin 1 cavity 

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

 kinds. Extrusions of such debris were often 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 summarized. 



Most of the larvae 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 mesenteries — 

 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 exoccelic chambers, not the entoccelic, as is usually the case. Two or three 

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

 simultaneously or in a successive manner. They were situated central to the first cycle 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 Siderastraea 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 exoccelic chambers, and with the primary exotentacles formed an outer 



