512 MR W. WEST AND MR G. 8. WEST ON 
peaty moors and bogs situated at a slightly higher altitude, and gained the pelagic ;, 
region of the lakes by the streams which drain the peat-bogs above. It must not be | 
supposed, however, that all the Desmids of the plankton are continually being 
replenished from the peaty areas of higher altitudes. This may be partially true of 
some species, but evidence does not support it in the case of others. Most of the 
species found in the Scottish plankton have also been found in the rocky pools of the 
peat areas, but some of them are so very scarce and so rarely met with in such 4 
situations, and at the same time are so very abundant in the plankton, that one can only 
conclude that the percentage of recruits arriving in the pelagic region of the lochs is — 
infinitesimal. A good instance of this is furnished by Stawrastrum Ophiura. Only 
once have we found this Desmid in the peat-bogs of the west of Scotland, and we have 
examined many of these situations much more minutely than the pelagic region of the 
lakes. Also, the late Dr J. Roy, although investigating Scottish Desmids for more than 
twenty-five years, never observed this species; and yet Stawrastrum Ophiura is one 
of the most abundant and characteristic Desmids of the Scottish Plankton. One can 
only infer from these facts that, so far as this species is concerned, recruiting for the — 
plankton from surrounding bogs must go on very slowly, and be more or less 
insignificant. 
Some of the plankton-Desmids have become slightly and others considerably 
modified by the assumption of this pelagic life, and the modifications are mostly in — 
the development of long spines and processes which give them both a greater floating 
capacity and a greater protection against the depredations of Desmid-eating animals. 
We have previously stated * that this increase in the length of spines and processes 
is chiefly exhibited by species of Staurastrum and Arthrodesmus. Other species of 
the genera Cosmarivum, Micrasterias, and Euastrum do not appear to have acquired — 
any special characters either for further protection or for augmenting their floating 
capacity. 
Dr C. Wesrnserc-Lunp t has recently remarked upon the abundance of Desmids” 
in the Scottish plankton, and suggests that many of these Conjugates are adopting a — 
pelagic life as they arrive in the lakes from the peat-moors. We think, however, that 
most of the Desmids of the Scottish plankton have existed under these pelagic 
conditions for a very long time, as there is every indication of this in the modifications 
some of them have undergone, and in the species and varieties which at present are 
only known to occur in the plankton. 
A point which should not be overlooked is the almost complete absence from the 
plankton of those Desmids which are most conspicuous and abundant in the surround- 
ing peat-bogs, and which are therefore the most likely species to be carried into the 
pelagic region of the lakes by the streams and rivers. It would appear from this that 
* W. West and G.S. West, l.c., p. 554. 
+ O. Wesenberg-Lund, “A Comparative Study of the Lakes of Scotland and Denmark,” Proc. Roy. Soc. Hdin., 
xxv., 1905. 
