ISO 



MEDUS.E OF THE WORLD. 



This large Mediterranean form is described in the synoptic table of species of Rathkca. 

 Gegenbaur, i854(Generationswechsel und Fortplanz.bei Medusen, etc.. p. 22, taf. 2,fign. 1-9), 

 raised the larva until it developed into a solitary hydroid, 0.5 mm. long, with 4 tentacles, 

 each beset with a row of nematocysts. A horny perisarc covered the hydrorhiza. 



The stages of segmentation were studied by Metschnikoff, 1886. The egg is o.ii mm. 

 wide and yellowish to white in color. The segmentation is total and equal, and results in the 

 formation of an elongated, ciliated, one-layered blastula with a large, central, segmentation 

 cavity. The ectoderm is formed by the cells which wander into the segmentation cavity by 

 ingression from the hinder end of the larva. 



The medusa is quite common at Naples, Italy, during the winter. The bell is barrel- 

 shaped with a flatly-rounded apex. It is 8 mm. high and 9 mm. wide. The marginal ten- 

 tacles are highly contractile. There are 10 to 13 tentacles in each of the 8 clusters. Each 



FK.. <)).-- Rtitlikea ftiscicitlalti. From living and preserved specimens, hy the author. Zoological Station, Naples, Italy. 



A, Side view of medusa with tentacles strongly contracted. R, Tentacle showing ocellus. (', Ahoral view of 

 manubrium. /), View of oral tentacle, contracted. 



tentacle bears an ocellus on its inner side. The oral tentacles branch dichotomously 7 times. 

 Whenever the medusa is disturbed hy a mechanical shock the oral tentacles suddenly expand 

 to their full extent, torming a network of filaments around the mouth, and thus it is seen that 

 through this fortuitous reaction the medusa captures its prey. Such a reaction is remarkable, 

 for in common with other medusae all other parts ot the animal contract when disturbed. 



The 4 gonads form horseshoe-shaped, swollen crescents with transverse furrows. Each 

 crescent surrounds the point of entrance of the radial-canal into the stomach and the horns 

 of the crescent are directed ahorally. There are thus 4 pcrraJial gonads. This being the 

 only instance of perradial gonods seen in the anthomedusa?. 



Rathkea octonemalis Maas. 



Ralhkea aclonrmalis, MAAS, 1905, Craspedoten Medusen der Siboga Expedition, Monog. 10, p. 12, taf. 2, fign. 1 1, 12. 

 ( ? ?) Lizzflla octella, HAECKFL, 1879, Syst. der Medusen, p. 84. 



Bell 5 to 6 mm. high, 4 to 5 mm. wide, with thick, gelatinous walls, bulging sides, and 

 flat top. There are 8 clusters of marginal tentacles. 7 to 9 tentacles in each ot the 4 radial, 

 and 5 to 7 tentacles in each of the 4 interradial clusters. These tentacles are not one-fourth 

 as long as the bell-height, and there is a black ectodermal ocellus upon the inner side of each 

 tentacle near the point of origin of the tentacle from the basal bulb. 



The 4 radial-canals and the ring-canal are straight, narrow, and simple. The stomach 

 is mounted upon a short, wide, conical peduncle. The mouth is surrounded by 4 clusters of 

 oral tentacles. Each cluster arises from a short basal stalk and branches dichotomously 

 4 to 5 times. The basal stalks of these clusters are so short and the forked branches so long 

 that the mouth appears at first sight to be surrounded by 8 clusters of tentacles. This impres- 

 sion is, however, erroneous, for there are but 4 clusters of oral tentacles. The mouth does not 



