1904 - 5 .] The Rhizopods and Heliozoa of Loch Ness. 613 
by the researches of Wesenberg-Lund and others, the problem is 
well on the way towards solution, the relation of the changes of 
form to the temperature and specific gravity of the water being 
almost demonstrated. 
The abyssal Rhizopods are unicellular animals of the simplest 
group of the Protozoa, extremely plastic, being simply Amoebse 
with shells, and are known to be extraordinarily variable, so much 
so that Leidy considered that different species, even species 
assigned to distinct genera, passed by insensible gradation one 
into another. In such animals it would not be surprising if the 
peculiar abyssal conditions as to light, temperature, pressure, etc., 
were to produce decided modification of form in individuals which 
grew up under these conditions. A dwarfing or increase of size, 
coupled with a relatively greater or less development of the 
chitinous plates of which the tests of most species are composed, 
might result in the production of very different-looking indi- 
viduals. The chitinous plates, according as they merely meet at 
the edges or overlap in greater or less degree, give rise to great 
differences of appearance, and some change of this kind might 
account for the difficulty Dr Penard found in deciding whether a 
Loch Hess Nebela should be assigned to N. Crenulata or to 
N. vitroea. 
The abyssal conditions being approximately similar in all deep 
lakes, the changes of form might always take definite directions, 
and this parallel modification might explain the occurrence of 
peculiar abyssal “ species,” apparently identical, in widely separated 
lakes. 
Further researches may modify our conclusions as to the abyssal 
fauna of our Scottish lakes ; it may prove that the abyssal 
Rhizopods are good species and varieties ; some means of dis- 
semination of abyssal species may be discovered, difficult though 
it is to imagine such ; the work of specialists may show that other 
groups than Rhizopods have abyssal species ; but, in the present 
state of our knowledge, it is easier to derive the abyssal fauna of 
our lakes from the shores of the lakes themselves, and from the 
surrounding country, than from those distant lakes in which alone 
similar abyssal conditions will be found. 
