OF MEDUS/E MADE BY WILLIAM KKITH BROOKS. 



9 



touching only slightly. After this the cleavage becomes remarkably irregular, recalling the 

 extraordinary condition observed by Hargitt in Pennaria. A solid morula is formed, which has 

 no central segmentation cavity and which resembles a loosely-connected mass of irregularly 

 grouped cells rather than an embryo of any metazoan. The cleavages follow one after another 

 at intervals of 20 to 30 minutes. 



As in Metschnikoff's Oceania annata and Margin's embryos of Pe nnaria, this irregularly 

 shaped morula gradually changes into an oval embryo, the surface of which becomes ciliated, so 

 that it swims upward from the bottom. This takes place at about 4 in the afternoon in eggs 

 which were laid between 5 and 6 in the morning. 



Rittenhouse finds that during this period, when the loose, irregularly shaped mass of cells 

 shapes itself into an oval embryo, the ectoderm and entoderm are formed. During this period 

 "the cell-boundaries are lost for a short .time and a syncytium is formed. This syncytial 

 structure is crowded with yolk-granules and nuclei are scattered throughout the protoplasm. 

 The nuclei soon become more numerous near the periphery and the cell-walls begin to appear" 

 between the peripheral nuclei. These peripheral cells are to become the ectoderm, which is 

 soon separated from the inner, structureless mass by the development of the mesoglcea. This 

 inner mass afterwards acquires cell-boundaries between its nuclei, and still later a central 

 cavity, the coslenteric space, develops; and thus the entoderm is formed. This coelenteric 

 cavity does not develop, however, until the larva is 48 to 60 hours old. The syncytium con- 

 dition in Turritopsis is much more complete than is seen according to Hargitt in Pennaria, or 

 in Bougainvillia, according to Gerd, 1892. 



Rittenhouse finds that during the earl)- stages of cell-division in Turritopsis the multipli- 

 cation is solely by mitosis, but that later, when the embryo becomes a mere irregularly arranged 

 mass of loosely-compacted cells, some of the nuclei divide amitotically. 



When about 50 hours old the elongate, oval larva ceases to swim through the water and 

 settles down upon its side on the bottom. The larva then becomes the hydrorhiza, or root, of 

 the hydroid, and the first hydranth arises as a bud from about the middle of its length. The 

 tentacles of the hydranth develop in indefinite whorls, with 4 tentacles in each whorl, the 

 oldest tentacle being nearest to the mouth. In the mature hydroid the tentacles appear to 

 be irregularly scattered rather than being arranged in whorls. 



Turritopsis should be reared under more natural conditions than those of the ordinary 

 laboratory in order to determine whether the remarkable, irregularly formed embryos described 

 by Rittenhouse be normal or merely the result of pathological states induced by adverse 

 conditions; but Miss Beckwith has recently shown that the cleavage of Pennaria is normally 

 irregular as is described by Hargitt. 



Rittenhouse finds that when the embryo is in the loose-celled, morula stage it may be 

 divided into two masses, each one of which produces a normal planula larva of small size. 



Genus WILLSIA Forbes, 1846. 

 Willsia brooksii sp. nov. 



Beautiful drawings of a young stage and also of the adult condition of this medusa 

 were made by the late Prof. William K. Brooks, while he was at Beaufort, North Carolina, 

 and were found among his unpublished figures, after his death. They were kindly pre- 

 sented to me by the Department of Biology of Johns Hopkins University for publication in 

 this work, and it seems but fitting that the species should be named in honor of the great 

 naturalist who discovered it. It is closely allied and possibly identical with the European 

 Willsia stellata, although the 3-rayed center of the stomach appears to distinguish it. 



In the young stage there are 6 simple, slender radial-canals, 60 apart. The bell-walls 

 are relatively thin and the bell somewhat higher than a hemisphere with a bluntly pointed 

 apex. The 6 tentacles are 5 to 6 times as long as the bell-diameter and have swollen, 

 nematocyst-bearing, outer extremities. 



In the mature medusa the bell is flatter than a hemisphere, thick walled, with a shal- 

 low bell-cavity. Twenty-four tentacles alternate with 24 exumbrella, nematocyst tracts 

 each with several clusters of nettling cells. The manubnum has 6 lips. Stomach 3-rayed 

 at center, but each ray forks, giving 6 ramuli, 6 primary radial-canals which bifurcate twice, 

 giving 24 terminal branches. Gonads extend along sides of stomach. The size and color 

 can not be determined from Professor Brooks's drawings. Found at Beaufort, N. C. 



