DISCOVERY REPORTS 



ISO 50 



NUMBER OF FIBRES 



Text-fig. 5. The percentage frequency of occurrence of muscle-fibre counts in the aggregate and solitary forms 

 of 5. fusiformis (black solid), S. thompsoni (sohd line) and S. gerlachei (broken line). 



125 



100 



S 75 



25-, 



30 



35 



40 



50 55 



DEGREES SOUTH 



bO 



65 



70 



Text-fig. 6. Individual muscle-fibre counts of specimens of the sohtary form of S. fusiformis (A), S. aspera{o), S. thompsoni (•) 

 and 5. gerlachei (a) plotted according to the latitude of the station at which they were taken. Note: observations for S. fust- j 

 formis and S. aspera at stations north of 30° S. are not included. 



The data for S. aspera are insufficient to show the percentage frequency of occurrence as has been 

 done for the other species. However, when the data plotted in Text-fig. 3 are compared with those 

 of Text-fig. 4 it is seen that in fibre number S. aspera Hes roughly within the same range as S. thomp- 

 soni. It is thus impossible to distinguish these two species on the basis of their muscle-fibre number 

 but it will be remembered (Table i, pp. 14 and 15) that they can be distinguished in both aggregate and 

 solitary form by differences in their muscle arrangement, and in addition, in the solitary form, by 

 external diiferences in serrations of the test. 



As mentioned above there is overlap between the fibre counts for some of the species, as, for 

 instance, between the solitary forms of S. fusiformis, S. thompsoni and 5. gerlachei (Text-fig. 5) and 

 it might be argued that 5. gerlachei, characterized by fewer muscle fibres and thinner muscles (see 



