IDENTIFICATION OF SPECIES 213 



this was not possible. However I have compared my specimens with those identified by Dr K. H. 

 Barnard and as there is agreement the specific name antarctica has been used. As previously 

 mentioned no attempt has been made to separate the two common species of Cyllopus. 



Euphausiacea. The Antarctic species of Euphausia have formed the subject of a previous Discovery 

 Report (John, 1936) and the descriptions and figures in this enable the adults of the various species 

 to be identified with confidence and usually with little trouble. 



Mollusca. The five species from this group that have been taken into consideration are, provided 

 they are not badly damaged, easily identified by their general appearance. The shells of the two species 

 of Limacina are rather fragile and are often broken in the samples but they are usually present in such 

 numbers, particularly L. balea, that sufficient complete specimens may be found. 



Tunicata. The only Tunicate considered here is Salpafusiformis f. aspera. This is readily identified 

 by the spinose ridges on the test. This characteristic is used by Thompson (1948) in a key to the 

 Australian pelagic Tunicata as the distinguishing feature between S. fusiformis fusiformis and S. fusi- 

 formis aspera. 



Excluding any possible errors inherent in the actual method of obtaining samples of the population 

 with the nets, it will be seen from what has been said above that there are two sources of possible error 

 in dealing with the plankton samples themselves. The first, the overlooking of smaller organisms, is 

 generally corrected by the examination of additional samples. The second, error in identification, has 

 been shown to be comparatively slight and usually to occur only in dealing with juvenile forms. Again, 

 where only presence or absence is being considered, this has little effect, as there must be few samples 

 containing nothing but juveniles of a species, and only one adult specimen is required for evidence of 



presence. 



DISTRIBUTION 



As in the section on phytoplankton, Figs. 4 and 5 show the frequency of occurrence of a genus or 

 species in each sector expressed as a percentage of the number of stations examined in that sector. 

 E.g. in the sector 0-20 E. Diphyes antarctica was present in twelve out of the sixteen samples examined 

 and is thus shown as having a frequency of occurrence of 75 %. 



When all the 230 samples had been examined it was found that eight species still showed gaps in 

 their distribution of between 20 and 6o° of longitude in certain areas, but it was thought that these 

 gaps might be caused by insufficient sampling of a rather sparse population and not by their complete 

 absence from those regions. Therefore additional samples were examined for the discontinuous species 

 only, and, with the exception of Eusirus antarcticus, all such gaps disappeared. Where samples have 

 been examined for only a few species these samples are not included under ' No. of samples examined ' 

 and the presence of a species is indicated in the histograms by a pecked line. 



The purpose of the histograms is to show diagrammatically the circumpolar continuity of all the 

 species whose distribution has been examined in this connexion, and this continuity is seen to hold 

 good for all, except in so far as Eusirus antarcticus shows a gap of more than 20 of longitude. 



The variations in the height of the histograms are of no great significance, for they merely show the 

 frequency of occurrence of a species in those samples which happen to have been chosen from 

 among those available in each 20 sector. The only reason why this percentage occurrence is indicated 

 in the diagrams is that it shows some species to be more nearly ubiquitous than others. Thus it may 

 be noted that the following species occur in over 80 % of the total samples examined : Sagitta gazellae, 

 Calanus propinquus, Eukrohnia hamata, Thysanoessa spp., Rhincalanus gigas and Calanus acutus. This 

 sequence represents the order of frequency in which they were found, Sagitta gazellae being present 

 in 96-0 % and Calanus acutus in 83-5 % of the samples. Taking the lower range of frequencies, Eusirus 

 antarcticus and Sibogita borchgrevinki occur in only 9-5 and 15-0% of the samples respectively. The 



