OF MEDUSAE MADE BY WILLIAM KEITH BROOKS. 19 



There are 16 subrectangular marginal lappets, with shallow median notches and rounded 

 angles. The septum between the ultimate branches of the radiating stomach-pouches in the 

 marginal lappets is twice as wide as the ultimate pouches themselves. The 4-sided throat- 

 tube is as long as the bell-radius. The 4 lanceolate lips or palps, with their complexly tolded 

 margins are each about 1.33 as long as the bell-diameter. Thus in an adult medusa with a 

 disk 49 mm. wide the palps were 68 mm. long. 



The bell has a rich rose-purple tinge; the gonads, the entodermal cores and the tentacles 

 being especially deep in this color. The warts upon the exumbrella and along the outer edges 

 of the palps are orange-brownish red. 



This medusa is abundant at times in the Mediterranean, especially in summer, although 

 large specimens are rarely seen in winter. It may be locally abundant during several suc- 

 cessive seasons and then vanish for years. For many years it was all but unknown in the 

 Bay of Naples but since 1900 it has been one of the commonest Scyphomedusae in th;s region. 

 It ranges widely over the warm regions of the Atlantic. 



The development has been studied by Krohn, Kowalevsky, Hamann, Goette, Hyde, and 

 Metschnikoff. Hamann, 1883, has made a detailed study of the development of the gonads, 

 and their structure has been described by the brothers Hertwig, 1878. They appear as 4 

 interradial, elongate ridges in the entoderm of the subumbrella. The entoderm forms a series 

 of follicles in which the sex-cells develop and then migrate into a gelatinous lamella between 

 the layers of entoderm. 



According to Metschnikoff, the egg is violet-brown and is laid between 12 and 2 in the 

 afternoon, in December, in the Mediterranean. Segmentation is total and nearly equal, and 

 a very large central segmentation-cavity is formed. The gastrula results from invagmation 

 at the hinder end of the body. The blastopore does not close, but forms the mouth of the 

 larva. Thus, according to Goette, 1893, the mouth is ectodermal and forms by invagination 

 at the hinder end of the larva, but the invaginated sac by no means fills the segmentation 

 cavity. The first pair of stomach-pouches arise from the entoderm and are 180 apart, then 

 follows an ectodermal pair 90 away from the first. The latter then develop 2 lateral pouches 

 each, and at a later period the entodermal pair each gives rise to 2 lateral pouches, thus giving 

 a larva with 6 ectodermal and 6 entodermal stomach-pouches; finally the ectodermal pouches 

 give rise to 4 new adradial pouches and the larva has 16 stomach-pouches 10 ectodermal 

 and 6 entodermal. There is thus a striking analogy between its development and that of the 

 scyphostoma of Aurelha, according to Goette. 



The external features of the transformation of the free-swimming larva into the medusa 

 have been studied by Krohn (1855), Kowalevsky (1873), etc. The mouth-end of the larva 

 become expanded and crater-like, with the mouth at summit of central cone of crater. The 

 depressed region around the cone becomes the subumbrella. The lappets, into which the 

 gastrovascular cavity is continued, grow out at intervals around the margin. The covering 

 of cilia is lost from the body of the larva and it begins to swim by means of rhythmical con- 

 tractions of its oral disk. Thus the free-swimming scyphostoma is converted into a medusa 

 without sTobilization ("see Goette, 1893). 



Reasoning by analogy from the excellent work of Hyde, 1894 (Zeit. fiir wissen. Zool., Bd. 

 58, p. 531), upon Aurellia, it is probable that only the subumbrella floor of the second pair 

 of evagmated gastric pouches is formed from ectoderm, their exumbrella sides being of 

 entoderm. (See also Hadzi's work upon Chrysaora.} 



