MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 527 



are very limited in iiuiiiher iind distrihiitioii. Only a few occur around the po.sitioii at which 

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

 constituents of the ectoderm are distinctly shown in larvie doubly stained with borax carmine 

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

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

 carmine, while the supporting cells and nematocysts arc 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 l)odies, staining blue, with the nucleus red, which in all probabilitj^ 

 rei^resent developing nematocysts. The nuclear zone is very sharplj^ 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 gland cells. 



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

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

 actual pole it is very prominent. What seem to be 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 delicate fibrils, especially well seen in transverse sections. At the actual 

 extremity the gland cells are less plentiful and nematoc3'sts are more numerous. 



Throughout the larva the mesoglcea scarcely attains any appreciable thickness, but appears 

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

 it is barely seen as a definite la3'er. Associated with its endodermal surface are nuclear 

 bodies which stain deeply in methyl l>lue, 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 mesoglcea of the mesenteries, and 

 maj' perhaps be concerned in the formation of the middle layer. 



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

 thellas 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 c(Blenteric 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 

 Edwai'dsian mesenteries are complete, and though the fifth and sixth pairs are j'et free from the 

 stomodieum 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 stomodsval 

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

 the incomplete pairs only makes its appearance some little distance below the stomoda?um, and is 

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

 for some distance, ))ut 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 ISOPHYLLIA DIPSACEA. 



(Pis. XVII, XVIII, figs. 12.5-128.) 



A colony of hophylUa was collected from which larvte were freely extruded from the 

 beginning. On their first appearance most of the larvte 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 larvaj were larger than 



