528 MEMOIRS OF TlIK NATIONAL ACADEMY OF SCIENCES. 



those usually met with in corals, measurino- fully 3 mm. in length. "When tir.st extruded they 

 were den.sely opaque, and the posterior end (oral) was deeply pigmented. An hour or two after 

 being set free some became greatly distended at the aboral pole, and as a consequence were 

 more nearly transparent; others, again, became swollen at the oral exticmity, the opposite end 

 remaining narrow. 



In the inflated larva represented in tig. 1^5 three pairs of mesenteries were already indicated, 

 all extending downward from the minute, circular, oral aperture. The mem])ers of one 

 pair of mesenteries extended nearly the whole length of the larva, and along their line of 

 attachment were umch darker and broader than the others. On one side of the pair were two 

 other faint mesenterial attachments, which continued but a short way down the polyp, while on 

 the other side was a third pair only just apparent, and having a still shorter vertical course. All 

 three pairs, however, start from the uppermost extremity of the polyp. 



Some of the larva? immediately on extrusion were preserved in formol, and others in 

 corrosive acetic, when they threw out a quantity of mucus, which resulted in the adherence of 

 minute foreign particles. The distended larvte nearlj' always collapsed during the process of 

 preservation. 



Transverse sections of tlie freshly extruded sjjecimens reveal that the larvie are practically 

 solid bodies, the interior being tilled with a compact vacuolated tissue, bearing numerous nuclei 

 and zooxanthelh*. Boundaries in the vacuoliir endoderm are indicated toward the middle, in 

 association with the mesenteries, and in the middle of some of the larvse there is a faint indication 

 that the endodermal tissue is beginning to break down, but as yet they are practically solid. 



Towai'd the oral extremity the endoderm is crowded with zooxanthelhe, which are only 

 spaiingly distributed elsewhere. An examination of the outer ectoderm reveals comparative I3' 

 few alga?, and these are scattered somewhat uniformly throughout the layer. The strong 

 pigmentation of the oral extremity, noticed among the external characters, is manifestly due to 

 the accunudation of zooxanthellw within the oral endoderm, not, as is more usually the ca.se, 

 to their presence in large numljers in the ectoderm. 



Both longitudinal and transverse sections through the stomodanuu indicate the absence of 

 any actual lumen in the tube, and the compact character of the interior of the larva, above 

 described, is not such as to suggest that the circulation of anj^ internal nutrient fluid had been 

 established up to the moment of li))cration. 



The uppermost sections through the oral extremity reveal the presence of three pairs of 

 mesenteries, all extending from the outer wall to the stomodi^um. The ventral pair, however, 

 is represented only by the merest rudiments, and the dorsal pair extends but a short distance; 

 neither pair stretches downward the full length of the stomodseum. The middle of the three 

 pairs is by far the most important; its members are inserted on the stomodwum throughout its 

 extent, and when they become free the edge is tipped with a mesenterial fllament which appears 

 as a deeply-staining tissue, wholly resembling that of the stomodteal ectoderm; the two are in 

 absolute continuity with one another, and in every way seem one and the same tissue. The 

 mesenterial tilanient extends nearly two-thirds the length of the polyp, and is very conspicuous in 

 sections on account of the deeply-staining character of its constituent cells. The other 

 mesenterial pairs present no indications of filaments. 



/. dijjsacea is of interest as showing tlie eai'ly stage at which the second and third pairs of 

 mesenteries are united with the stomodanim; indeed, they seem to originate at the angle between 

 the wall and .stomoda?al invagination, and thence grow down the colunui and the stomodteum. 



The ectoderm is characterized liy numerous large clear gland cells, which give out their 

 mucus when the larvic are preserved. Many large nematocysts are also present, and the aboi'al 

 extremity displays a strongly developed nerve layer. 



LARVA AND YOUNG POLYPS OF YkXW FRAGUM. 

 (Pis. XIII-XV, figs. 96-116.) 



The polyps of several colonies of this species collected around Port Henderson, eaidy in 

 April, were charged with larv;e, which were extruded singly from tiuu> to time. Occasionally, 

 an imfertilized egg would also appear. The larvaj could be seen through the transparent walls 



