MORPHOLOGICAL CHARACTERS OF TAXONOMIC IMPORTANCE 17 



In Text-fig. 3 a and b individual counts for both forms of S. fusiformis and S. aspera are compared 

 while the mean values are given in Table 3. Compared to the rather limited data for S. aspera the 

 fibre counts for S. fusiformis show relatively little scatter about the mean of 51 -Si 1-4 for the aggregate 

 and 29-4± 1-4 for the solitary form. It is also evident, for all species, but particularly in the case of 

 S. fusiformis, that there is no increase in the number of fibres per muscle as the animal grows in size. 

 The number of observations for S. aspera, particularly of the solitary form, are limited and so the 

 means of 194-1 for aggregate and 1007 for solitary forms must be considered as approximate. Even 

 so there is no overlap between the counts for S. fusiformis and S. aspera and the difference between 



Table 2. Individual muscle-fibre counts in the solitary and aggregate forms of three of the four species 

 showing (a) the variation in fibre number between muscles, (b) the variation in fibre number between the left 

 and right sides of the same muscle in Salpa fusiformis and (c) the variation in the number of muscle fibres 

 from species to species. (S. aspera is not included because counts of individual muscles are not available 

 for the solitary form) 



Table 3. The variation in the mean number of muscle fibres in the four species, with the standard deviation 



and number of observations for each species. Standard deviations have not been given for S. aspera because 



of insufficient data for the solitary form 



Mean number of Standard 

 muscle fibres deviation 



SoHtary (M. IV) 



294 4-8 



100-7 — 



93-2 13-8 



48-8 7-3 



Aggregate (M. I-VI) 



51-8 4-9 



1941 — 



1777 187 



128-8 II-3 



S. fusiformis 

 S. aspera 

 S. thompsoni 

 S. gerlachei 



Number of 

 observations 



48 



9 

 92 

 67 



S. fusiformis 

 S. aspera 

 S. thompsoni 

 S. gerlachei 



45 



29 



126 



50 



them as shown in Text-fig. 3 and from the data of Table 3 is most striking. Text-fig. 3 also illustrates 

 the great difference in the maximum size attained in the two species, at least as represented by the 

 specimens available, S. aspera at its maximum being two to three times larger than S. fusiformis at 

 comparable stages of maturity. 



Text-fig. 4 shows the data for S. thompsoni and S. gerlachei. Comparing Text-fig. 4 with Text-fig. 3 

 it will be seen that S. fusiformis in both its forms has fewer fibres per muscle than any of the other 

 species, and does not attain the relatively large size of the other species. The data plotted in Text-fig. 4 

 for both forms of S. thompsoni and S. gerlachei show considerable scatter in the observations. Never- 



