FRESH-WATER PLANKTON 
345 
shape, but still I think all the different forms should be included 
under this species s. str. A very variable character is the length 
of the mucrones ; usually the mucro is long in the young individuals, 
and with age it becomes shorter and shorter, in some few cases almost 
disappearing in the adults. Often the long mucro of the young is 
notched ventrally (see Plate XIV. fig. 9), but at every moult the notch- 
ing becomes less marked, and in the adult there is usually no trace of 
such notches, though occasionally they persist: on the other hand, many 
long-spined young have no indication of notching. On examining 
the late embryos while still within the brood-pouch, we find that 
there the mucro reaches its highest development, relatively at any 
rate, and there too the notching is most conspicuous ; in such embryos 
the mucrones function as hooks for keeping the shell valves approxi- 
mated, the two hooks crossing each other, either hook folding over 
the opposite valve (see Plate XIV. fig. 15). And I suspect that the 
notching of the mucro has some reference to the same function. Now, 
seeing that the mucro of the adult Bosmina ohtusirostris varies 
considerably in length in the various lochs, and is frequently much 
reduced in size with age, we may suppose that it serves no very 
important function in the adult, and it seems probable that the 
above-mentioned is one of its chief functions, though doubtless the 
long mucrones of the young are useful as balancing organs. 
In the island of Lewis, Bosmina ohtusirostris is usually abundant 
in any loch. The adult is of large size, sometimes almost 1 mm. long, 
the mucro included ; the dorsal contour is rounded, the body is 
relatively high, the post-dorsal angle is obtuse, sometimes so much so 
that it almost disappears, the brow is protruding, the eye rather 
large, the rostrum is short and straight, the mucro is short, and the 
series of spines on the abdominal claw includes nine or ten spines. 
In the young, however, the body is relatively much more elongated, 
the posterior half of the dorsal contour is straight, the post-dorsal 
angle is less obtuse, the brow is not prominent, the rostrum is curved 
and long, the mucro is long, and the abdominal claw has only 
about four or five spines. Such is the Bosmina of Loch a' Chlachain 
(Plate XI fig. 11), Loch Bodavat, and others. 
In other lochs of I^ewis (Langavat, Trealaval, Skebacleit, etc.) we 
have very much the same form, differing, however, in that the rostrum 
is still shorter, but the mucro is long ; in the newly hatched young 
the long mucrones are notched, and the notching persists even in the 
adult specimens, though it is not very marked. Between the forms 
with short mucro and those with long mucro there is every grade of 
intermediate. 
In Loch Scaslavat the Bosmina was small, and it had a purple 
coloration ; it belongs to the same variety, however, as the above. 
