6 "TERRA NOVA" EXPEDITION. 



the aljseuce 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, Sonth 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 

 difi"erent 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 gaini, 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 

 (190'2) 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 Brachyura. 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 eff'orts 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 



