74 



MR. G. ARNOLD AND DR. C. L. BOULENGER ON 



to the umbrella for a short distance. The nematocysts are similar 

 to those of L. tanganicce. 



Nettle-Ring. — As in the type specimens of L. rhodesice, the 

 nettle-ring is comparatively narrow and thickened and folded 

 round the bases of the tentacles in such a way as to form 

 structures resembling the tentacle-bulbs which occur in so many 

 craspedote medusae. These "tentacle-bulbs" are particularly 

 conspicuous in the specimens from the Norquane River : this is 

 probably due, in part, to tbe great contraction of the umbrella- 

 margin, which is thus thrown into folds. 



As the structure of the nettle-ring and other organs of the 

 umbrella-edge forms the chief distinguishing character between 

 L. rhodesice and L. tanganicce, it seemed important to ascertain by 

 means of sections whether the peculiar appearance of the tentacle- 

 bases in the former species is due entirely to this folding of the 

 nettle-ring whilst the medusa is in a contracted condition. A 

 series of sections was cut in a longitudinal horizontal direction 

 through the umbrella-margin, and such sections show quite clearly 

 that, although the nettle-ring is a continuous structure, it is con- 

 siderably thickened at the base of each tentacle, these thickenings 

 forming the characteristic basal swellings which resemble the 

 tentacle-bulbs of other medusa?. 



In its histological structure the nettle-ring of L. rhodesice is 

 precisely similar to that of L. tanganicce as described b}^ one of 

 the authors in a previous communication (12). 



Text-fieTire 2. 



Sense-organs of Limnocnida rhodesice (A) and L. tanganica (B) viewed 

 under the same magnification. X 170. 



Sense-Organs. — The sense-organs are only slightly less numerous 

 than the tentacles : for instance, in the case of an individual with 

 96 tentacles, 84 of these Organs were counted. This is due to 

 the very definite relation between the arrangement of the sense- 

 organs and that of the tentacles in this species : a pair of the 

 former being situated at the base of each of the larger tentacles 

 near the velar margin of the nettle-ring, whilst a single sense- 

 organ occurs in a similar position at the base of each of the other 

 tentacles with the exception of the smallest, which are without 

 these organs. 



In the description of the Hunyani River specimens of L. rho- 

 desice (14). mention was made of the fact that the sense-organs 



