G 
“TERRA NOVA” EXPEDITION. 
the absence of Decolopoda from the extensive Ross Sea collections points to a restricted 
area of distribution for this genus, the two species of which have hitherto been taken 
only at the South Shetlands, South Orkneys, and Graham Land. A similarly limited 
range is more or less probable for several less conspicuous species, but cannot be 
regarded as established until a great deal more collecting has been done. 
V.—VARIATION AND SPECIFIC CHARACTERS. 
Prof. G. H. Carpenter (1907, p. 95) writes: “Recent systematic work on the 
Pycnogonida has brought home to students that a great plasticity of structure 
characterises this group, and that in large genera it becomes increasingly difficult 
with advancing knowledge to form definite specific diagnoses. The publication of 
new specific names is therefore attended with more than usual risk, but the very fact 
that variation is so wide makes the careful study of forms from any new locality of 
special obligation and interest to the naturalist.” Prof. Carpenter’s words have special 
weight as coming from a zoologist experienced in the systematic study of many widely 
different groups of Arthropoda ; nevertheless, it may be doubted whether, in this 
respect, the Pycnogonida differ so greatly from other large groups of marine arthropods 
as these words seem to suggest. The general impression gained from the study of such 
a collection as the present is much the same that would result from examination of 
many groups of Crustacea, for instance. Certain genera and families present large 
numbers of minutely separated species, the distinguishing characters of which have at 
least the appearance of inconstancy ; while other groups are composed of few species 
easily and sharply defined by characters that are relatively invariable. To the first 
category plainly belong many of the species of Colossendeis, Nymphon, and Achelia 
discussed below. On the other hand, we have such forms as Pycnogonum gain!, the 
sole representative of its genus in the Antarctic, which ranges from Graham Land to 
the Ross Sea and Wilhelm Land without perceptible variation in its specific characters. 
The question, however, deserves further study, since there are some reasons for 
expecting the Pycnogonida to be especially inclined to specific instability. Doderlein 
(1902) attributes great importance, among the factors favouring the development of 
local races, varieties, and species in any group of animals, to the lack of “ Vagilitat " 
or power of wandering, and Doflein has attempted to show how this “ Doderleinsche 
Prinzip ” applies to the case of the deep-sea Braehyura. Now, there are probably 
few groups of marine Arthropoda that are less “ vagile ” on the whole than the 
Pycnogonida. Although some species have the power of swimming in the adult state, 
their efforts seem to be awkward and inefficient, and none of the larvae are better 
adapted for locomotion. Whatever may be the result in comparison with other 
Arthropoda, the application of the principle does not meet with very encouraging 
