i6 DISCOVERY REPORTS 



var. orientalis. He concludes (p. 28) that in the soUtary form of T. democratica there is a definite 

 tendency for the number of fibres per muscle to increase with age although his data (p. 32) show no 

 such increase in the aggregate form nor in the solitary form of T. democratica f. orientalis (p. 36). 

 Further data for specimens of both type and variety of T. democratica from waters of Algeria are 

 given by Bernard (1958) which confirm the results of Tokioka (1937) and Sewell (1953). Berner (1954) 

 has drawn attention to the marked difference in fibre number in Ritteriella picteti and R. ambionensis, 

 two species which otherwise appear very similar. 



Berrill (1950) has discussed fibre number in general and has shown (p. 592) that the aggregate form 

 of a species not only has fewer muscle bands than the solitary form but that they are individually 

 narrower owing mainly to differences in the number of constituent fibres and in part to differences 

 in the diameter of individual fibres. His data show not only that the solitary form may have three to 

 four times as many fibres per muscle as its respective aggregate form but, what is more important, 

 that there may be a considerable difference in the counts for different species. The following are some 

 of the data given for individual muscle bands : 



S. fusiformis solitary, 30-33 fibres aggregate, 7 fibres 



T. democratica solitary, 10 fibres aggregate, 3 fibres 



S. maxima solitary, 55-65 fibres aggregate, 20 fibres 



Furthermore, in every species examined by Berrill (p. 593) the number of fibre rudiments first 

 discernible in the embryo or bud is also the final number to be found in the largest solitary or aggre- 

 gate individual; in other words there is no increase in number with age. 



In the course of checking Berrill's counts, specimens from the Southern Ocean, thought to be 

 iS. fusiformis aspera, were examined and it was immediately apparent that both aggregate and solitary 

 forms had at least three times more fibres per muscle band than the number given for S. fusiformis 

 by Berrill. From the subsequent examination of specimens from as wide a geographic range as 

 possible it was obvious that on the basis of fibre count alone three of the four species within the 

 fusiformis group could be distinguished. 



The data for the aggregate and solitary forms of S. fusiformis, S. aspera, S. thompsoni and S. gerlachei 

 are shown in Text-figs. 3 and 4, in which the fibre count for each specimen is plotted against its body 

 length. Before discussing these results it is necessary to consider the treatment of the data, and the 

 reasons for using the number of fibres in M. IV in the solitary form and the sum of the counts of 

 M. I-VI in the aggregate form. 



Specimens of the solitary form of all species, except S. fusiformis, in which all the nine body muscles 

 were sufficiently well preserved to enable each to be counted, were comparatively rare. Thus while it 

 was realized, as the data in Table 2 show, that the number of fibres varies slightly from muscle to 

 muscle and even from side to side, it was decided to count the number of fibres in one muscle only 

 of each specimen. Body muscle IV proved convenient for this purpose as it is easy to locate and 

 counts were made on the dorsal surface as near to the middle line as possible. A further reason for 

 using one muscle is that S. thompsoni has a very high fibre count, ranging from 70 to 130 fibres per 

 muscle, which would make the counting of all nine muscles in each specimen unduly tedious and 

 quite unnecessary for specific identification. 



For the aggregate form total counts, that is the sum of the number of fibres in each body muscle 

 (counted along the same side if possible), are used because, as the data in Table 2 show, there is 

 considerable variation in fibre number from muscle to muscle. While such variation is insufficient 

 to mask any difference based on single muscle counts between S. fusiformis and the other species, it 

 is great enough to make the differentiation of 5. thompsoni and S. gerlachei obscure. 



