FresJnvater Medusa of Lake Tanganyika. 271 



Hum of flattened polygonal cells. The velum is well developed 

 in some specimens, but not so conspicuous in others. The 

 circular muscles of the velum (fig. 5, m.v.) are arranged 

 on the inner side in separate concentric bands, varying in 

 number according to the breadth of the velum. The muscle- 

 bands near the free margin are always more highly developed 

 than those situated near the attachment of the velum. 



The gastrovascular system differs from that of all other 

 Medusae hitherto described in the relative size of its parts. 

 Tlie mouth and stomacli are both circular, widely open to the 

 exterior, and of so great a diameter — two thirds that of the 

 umbrella — that the lips of the mouth probably never com- 

 pletely close the stomach in the adult animal. In all the 

 specimens examined the central portion of the shallow stomach- 

 cavity is almost completely filled with the large central lens- 

 shaped boss of the umbrella (fig. 4), so that the stomach is 

 reduced to a circular trench all round the central boss inside 

 the manubrium, and can hardly be said to possess any central 

 cavity at all. It might be suggested that this extraordinary 

 condition is merely due to post-mortem changes j but the 

 fact that the relative dimensions of the parts do not vary 

 appreciably in any of the specimens examined, seems to me to 

 be sufficient reason for regarding the condition described as 

 the normal one, at any rate when the animal is not feeding. 

 A possible explanation of this curious dilatation of the mouth 

 and stomach was suggested to me by Mr. G. C. Bourne. It 

 is obvious that any increase in the diameter of the mouth and 

 stomach would involve a corresponding increase in the circum- 

 ference of the manubrium, which, as will appear in the sequel, 

 is the bearer of the reproductive organs. The large size of 

 the mouth would appear, then, to be correlated with an enlarge- 

 ment of the area upon which the reproductive organs, sexual 

 or asexual, are developed. 



The slightly thickened margin of the mouth is quite 

 simple, without processes of any kind. The radial canals are 

 four in number in the majority of individuals ; but instances 

 of the occurrence of five or six seem to be by no means rare, 

 specimens with six being rather more numerous than those 

 with five. The radial canals radiate from the stomach in a 

 more or less horizontal plane, but assume a more vertical 

 direction just before they enter the ring-canal, in consequence 

 of the inflexion of the margin of the umbrella. The ring- 

 canal varies in size in different individuals ; it is lined by an 

 epithelium of small squarish cells, similar to those lining the 

 radial canals. On the outer side of the canal, however, these 

 cells are in very great numbers, forming a lobulated mass 



20* 



