MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 527 



are very limited in number and distribution. Only a few occur around the position at -which 

 tlic oral aperture will be formed, as noticed among the external characters. The cellular 

 constituent- .if the ectoderm are distinctly shown in larvse doubly stained with borax carmine 

 and methyl blue, and the enlarged liases of the cilia are clearly distinguishable. Apparently on 



preservation none of the gland cells extruded their contents, and these arc now stained with the 

 carmine, while the supporting cells and nematocysts are stained blue. Most of the gland cells 

 have finely granular contents, but others are clear, and they may extend nearly the whole width of 

 the layer. The nematocysts are small and not very numerous. In the deeper parts of the layer 

 are numerous clear, elongated bodies, staining blue, with the nucleus red, which in all probability 

 represent developing nematocysts. The unclear zone is very sharply limited on its outer margin, 

 and the peripheral zone, wholly devoid of nuclei, occupies nearly one-half the thickness of the 

 whole ectodermal layer, made up for the most part of the swollen eland cells. 



About midway down the column wall an ectodermal nerve layer begins to appear. At first 

 very feeble, it becomes better developed as the aboral extremity is approached, until at the 

 actual pole it is very prominent. What seem to he delicate nerve fibrils extend vertically from 

 the mesoglcea, and then unite in a broad meshwork; under high magnification the mesh exhibits 

 the cut ends of very dedicate fibrils, especially well seen in transverse sections. At the actual 

 extremity the eland cells are less plentiful and nematocysts are more numerous. 



Throughout the larva the mesogloea scarcely attains any appreciable thickness, hut appears 

 as a mere dividing lamella between the ectoderm and endoderm; even in the mesenteries 

 it is bandy seen as a definite layer. Associated with its endodermal surface are nuclear 

 bodies which stain deeply in methyl blue, and exhibit somewhat of a punctate character, as if 

 nuclei in some mitotic phase, but, owing to their minuteness, no further details can be made 

 out. Similar appearances occur also in connection with the mesog-kea of the mesenteries, and 

 may perhaps be concerned in the formation of the middle layer. 



Tin' endodermal cells are spheroidal or polygonal, and almost completely vacuolated. Zooxan- 

 thellse crowd the endoderm cells toward the oral end of the larva, and are sparsely distributed 

 throughout: they show a slight tendency toward a restriction around the periphery of the 

 endoderm. Although compact, the endodermal tissue presents a definite series of internal 

 boundaries associated with the mesenteries, which indicate the lines along which cavities or 

 passages will be formed when the larva becomes distended and the ccelenteric cavity is ultimately 

 established. 



The freshly extruded larvse of Agaricia are somewhat exceptional in the degree to which 

 the mesenterial development has already proceeded. In all the specimens examined the 

 Edwardsian mesenteries are complete, and though the fifth and sixth pairs are yet. free from the 

 stomocheum they extend vertically nearly the whole length of the larva. The dorsal directives 

 cease aborally a little in advance of the remaining three pairs of complete mesenteries. 



Mesenterial filaments are strongly developed on the Edwardsian mesenteries, and less so on 



the two incomplete pairs; in the former they are in direct continuity with the stomodseal 



ectoderm, but it is obvious that this can not be the case with the latter. The tilamental tissue on 



the incomplete pairs only make- its appearance some little distance below the stornodauun. and is 



never so strongly developed as on the other mesenteries. On these the filaments are often weak 



for some distance, but toward their lower termination they become greatly developed, at least on 



the mesenteries of the first and second developmental pairs. Here the}' are very conspicuous 



objects in sections, and developing nematocysts and gland cells in various stages can be found. 



The filaments have already very definite boundaries distinguishing them from the rest of the 



endodermal tissue. 



LARVA OF ISOI'IIYLI.IA DIPSACEA. 



(Pis. XVII, XVIII, flgs. 125-128.) 



A colony of Tsophyllia was collected from which larva' were freely extruded from the 

 beginning. On their first appearance most of the larva' were rod-shaped, but others were 

 pear-shaped; at first the former would crawl along the floor of the vessel in a worm-like 

 manner, while the others would swim freely throughout the water. The larvse were lararer than 



