MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 525 



distributed, or, as in Agaricia, are more restricted toward the oral extremity and periphery 

 (lig-. 165). 



In larvffi of Agaricla and Isophyllia the endodermal tissue is in its most compact condition, 

 and in both transverse and longitudinal sections slits or lines of demarcation are present, which 

 limit one portion of the tissue from another. Along the margin of the slits the cells have more 

 contents, and the boundaries of the mesenterial filaments and more central part of the mesen- 

 teries are also shown, the tissue appearing as a matrix in which these organs are embedded. 

 The slits thus serve to delimit a parietal, mesenterial, stomodasal, and middle endodermal tissue 

 (PI. XVIII). In some larvje the slits are represented bj' wider, more definite spaces, especially 

 in the stomodieal region, while below the stomodseum the middle endodermal tissue can be seen 

 in process of breaking down. Onl3' the middle tissue, however, undergoes disorganization; 

 that lining the wall and mesenteries persists as a thickened mass for a long time. Where the 

 process has continued for some time the middle of the larval cavity is occupied bj- organic debris, 

 comprising granules of various kinds, fragments of cell walls, and zooxanthella; (PI. XIV, 

 fig. 112). This is afterwards extruded by the larvre shortly after the establishment of the oral 

 aperture (PI. XIII. fig. 96). 



For a long time the parietal and mesenterial endoderm remains enormouslj' thickened, arranged 

 in high vertical ridges, all the cells of the same vacuolated character, in both respects difi^ering 

 from the epithelium of the mature polyps. G. von Koch (1897), in his paper on the development 

 of CaryoplnjlJia cyathxs, and later in "Das Skelett der Steinkorallen," has drawn particular 

 attention to the parietal thickenings of the endoderm in larvae of this and a somewhat later stage. 

 As a rule the endodermal thickenings assume a definite form and relation with the mesen- 

 teries, which varies as the latter increase in number. In the section of the larva of F. fragum, 

 represented in fig. 116, they are ten in number, two axial thickenings and four bilateral pairs; at 

 a later stage another pair will be formed, and ultimately a thickening will occur in each of the 

 twelve mesenterial interspaces. Von Koch (1896) has found similar endodermal swellings in the 

 corals Astroides, Balano2}hyUla, and Caryophyllia, and in the Hydroids Coryne and Tuhularia; 

 Haddon (1890) figures exactly similar structures in the larva of EnpKyllln. 



The thickenings correspond with the positions which later will be occupied by the calcareous 

 septa, and von Koch has applied to them the term Prosepta (Vorsepten). It is not to be assumed 

 that the}' in anj' way represent the septa, or are concerned in their formation, for they are just 

 as well developed in Actinians, e. g. , Lebrunia, which never form a skeleton. From their 

 structure and arrangement, von Koch supposes that in the larva the endodermal thickenings 

 function as elastic supporting organs; that they are the physiological predecessors of the septa. 

 Morphologically they are seen to be the remnants of the vacuolated endoderm, which, at an 

 earlier stage, practically filled the interior of the larva. 



When the larva settles the thickenings still persist in the lower region, and extend intermesen- 

 terially along the base and for some distance up the column, as shown in the section of 

 Manicina (fig. 137). The septal invaginations of the ectoderm arising later are formed within 

 the prosepta, so that the skeletotrophic endoderm is greatly thickened from the beginning. 



Apparently in the larval prosepta we have the precursors of the enormously thickened 

 vacuolated skeletotrophic endoderm, already described as characteristic of the lower aboral region 

 of a great number of corals. 



The prosepta are thus the persistent representatives of the endodermal tissue, which at an 

 earlier stage completely occupied the internal cavitj- of the larva\ The middle portion of this 

 tissue becomes disintegrated, and the debris exti'uded from the larvw, while the peripheral portion 

 persists, becomes associated with the skeletal ingrowths, and undergoes more or less histological 

 alteration. 



In the larva of the Actinian Lehrunla (1899) I have already described a somewhat similar, 

 nearly solid condition of the interior, and in this case the tissue of the earliest larvae showed 

 definite narrow spaces, which were regarded as indicating a primitive ccelom. These spaces 

 correspond with the narrow slits and limitations met with in the freshly extruded larvaj of 



Vol. 8— No. 7 9 



