282 DISCOVERY REPORTS 



perradial corners of the quadrangular mouth as four distinct oral arms (Plate XIX, fig. i). 

 The simple mouth rim has no free edge below the oral arms, but these are direct pro- 

 longations of the mouth rim into tentacle-like lips with clusters of nematocysts ; they are 

 not inserted some distance above the mouth rim as in Lizzia and Bougainvillia. These 

 oral arms are unbranched in all stages, and have on their inner side clusters of nemato- 

 cysts. The clusters vary in number in accordance with the age of the medusa. In the 

 early stages there are only three clusters present: two lateral, opposite or nearly opposite 

 each other, and one terminal cluster. The terminal cluster is always present in every 

 stage, and the increase in number occurs only in the lateral clusters. Some of the large 

 adults have oral arms with eleven clusters: a terminal cluster and ten lateral clusters 

 arranged in two longitudinal rows. The oral arms are rather short, about the same 

 length as the stomach, and do not extend so far down as the velum. 



Medusa buds (Plate XIX, fig. i) are present in the early and intermediate stages and 

 sometimes also in the adult ; they are placed interradially on the walls of the stomach. 

 The little medusa ready for liberation has three tentacles in each of the eight groups, and 

 its oral arms have a small terminal cluster of nematocysts and the first lateral pair is just 

 appearing. As a rule asexual reproduction by gemmation ceases before the gonads are 

 fully ripe, but occasionally medusa buds are still present at the same time as the gonads 

 are nearly or fully mature. 



In the male the gonads are divided by four narrow perradial furrows; they form 

 roundish swellings occupying the whole of the interradial space and, except for the 

 furrows, completely cover the stomach. In the female the ova also completely cover 

 the stomach, but the perradial dividing furrows are not visible. Several specimens show 

 that the ova remain attached to the stomach until the planula stage is reached. The 

 largest planulae are at the upper end of the stomach and are held by a slender thread. 



The different stages in the collection give a good idea of the development of the 

 tentacles. The early stages have three tentacles in each of the eight groups. The inter- 

 radial groups do not increase the number of their tentacles beyond three, except when a 

 variation occurs, affecting one or two groups, but not all the groups. The tentacles in 

 the perradial groups increase in number, and the maximum is five in each group. 



The compound basal bulbs contain a dark brown or black pigment, but there is no 

 definite ocellus. 



The Falkland species is very much like Rathkea octopimctata (M. Sars), probably the 

 only other existing species of the genus Rathkea as this is now defined (see Hartlaub, 

 191 1, p. 228), and it has the same number of tentacles and the same arrangement of the 

 tentacles. It also passes through an almost similar course of development. The shape 

 of the oral arms marks the only difference between these two species: in R. formosissima 

 the oral arms are simple and undivided and have a terminal cluster of nematocysts ; in 

 R. octopimctata their terminal end is bifurcated, carrying two clusters of nematocysts; 

 in both species there are also a number of lateral clusters, one or two pairs on each of the 

 oral arms in R. octopunctata, four or five pairs in fully developed specimens of R. for- 

 mosissima. 



