ON A FRESHWATER MEDUSA FROM RHODESIA. 
435 
with distinct tentacle-bulbs and regularly arranged sense- 
organs. 
When I first examined the Rhodesian medusse these char- 
acters appeared very striking, and seemed of considerable 
importance; the differences, however, seemed much less 
conspicuous when the umbrella-margins of the two species were 
examined side by side. I, therefore, made a careful study of 
the marginal region of L. tanganicae, and have figured a 
small portion of the nettle-ring and neighbouring organs 
of this species for comparison with the similar figure of 
L. rhodesiae (vide PL 42, figs. 4 and 2). 
The nettle-ring of L. tanganicae has been described as 
forming a broad, continuous band along the margin of the 
umbrella; this is certainly quite correct, but it is to be noted 
that this band, although continuous, is not of equal thickness 
throughout. The axes of the larger tentacles can be seen 
through the nettle-ring (PL 42, fig. 4), and on the sides of these 
the latter structure is considerably thickened, so that, although 
distinct tentacle-bulbs are not formed, they are at least 
indicated, a point which seems to have escaped the notice of 
previous students of this medusa. Moreover, these have 
stated their inability to make out any relation between the 
arrangements of the sense-organs and tentacles in L. taug- 
anicae. A careful examination of the umbrella margin of 
this species has convinced me that such an arrangement does 
exist although it is somewhat obscure. 
In an adult medusa the marginal sense-organs are so 
numerous as to form an almost closed riug round the circum- 
ference of the umbrella; occasional gaps are, however, left. 
Now each of these gaps is invariably situated opposite the 
base of one of the larger tentacles, and when the sense-organs 
on either side of it are examined it becomes obvious that they 
form a pair belonging to the tentacle in question (PL 42, fig. 
4 a, a and b, b). 
The crowded sense-organs between such pairs roughly 
correspond witli the smaller tentacles ; the latter are, however, 
somewhat more numerous because sense-organs are not 
