MORPHOLOGICAL CHARACTERS OF TAXONOMIC IMPORTANCE 21 



p. 23) than S. thompsoni but otherwise identical in all other morphological features, represents an 

 extreme of variation. On this assumption one might expect there to be some sort of gradation in 

 muscle fibre number between the limits of the animal's distribution. In Text-fig. 6 the data (for 

 stations south of 30° latitude) for the solitary form of each species have been plotted against the 

 latitude of the station at which they were taken, and it is apparent that no such gradient exists in the 

 character of muscle-fibre number from north to south, the change from ' many fibres ' to ' few fibres ' 

 being particularly abrupt. It will be noted that S. aspera on rather limited data has similar counts to 

 S. thompsoni and so, as mentioned above, this character cannot be used to distinguish these two species. 



The Arrangement of the Body Muscles 



Unlike the external character of the test great reliance has been placed by many previous workers 

 on muscle arrangement for differentiating salp species, and Streiff (1908) and others have given 

 detailed accounts of the body, atrial and oral muscles of most species. While the arrangement of 

 the atrial and oral muscles is both complex and difficult to study, the body muscles are relatively 

 simple and easy to see, so making them useful as taxonomic characters particularly as their arrangement 

 within a species is more or less constant. Sewell (1926, p. 67) has remarked that large variations do 

 occur both with regard to the number of muscle bands and the connexions between them, and warns 

 that the muscles are delicate and so their arrangement can be altered by rough handling. As an example 

 Thompson (1948, PI. 73, fig. 3) shows the variation that can occur in the connexions of the body 

 musculature of the solitary form of S. cylindrica. Nevertheless, in most species muscle arrangement 

 may be combined with other characters, and it remains one of the most useful taxonomic features 

 (Yount, 1954). 



In three of the four species described, S.fusiformis, S. thompsoni and S. gerlachei, there is the same 

 basic arrangement of body muscles. In the aggregate form there are six dorsal body muscles which 

 curve laterally to the ventral surface where they are interrupted. The six muscles are disposed in two 

 groups with dorsal fusion of M. I-IV and M. V-VI ; and lateral fusion of M. IV and V. The solitary 

 form has nine body muscles which like the aggregate are interrupted ventrally. There is dorsal fusion 

 of M. I-III and M. VIII-IX, while M. IV-VII are parallel to each other. 



S. aspera (PI. I) has the same basic muscle arrangement as that just described except that in the 

 aggregate form (PI. I b) M. IV-V converge but do not join laterally and in the solitary form (PI. la, c) 

 M. VIII-IX do not join but like M. IV-VII are parallel or nearly so. 



It will be shown in the section on distribution that S.fusiformis and S. aspera are sympatric and so on 

 occasions are taken together in the same net sample. From the foregoing it will be realized that setting 

 aside other differences such as test character and muscle-fibre number there is no striking difference 

 between S. fusiformis and S. aspera, except in the arrangement of body muscles in the solitary form, 

 and as the two species can occur together this has contributed to the confusion, particularly with 

 regard to the aggregate form of the two species, described in the historical section. Herdman (1888), 

 for example, differentiated the solitary form of S. echinata on the basis of serrated test and M. VIII-IX 

 being parallel, but failed to associate it with specimens of the aggregate form in his collection in which 

 M. IV-V did not join laterally, regarding the latter as variations of S. runcinata {= S. fusiformis). 

 There has m fact been little significance attached to the lateral separation of M. IV-V in the aggregate 

 form, such separation being regarded as a mere variant (Herdman, 1888) or a character associated 

 with old age (Ritter, 1905) : the latter in spite of Apstein's illustration (18946, PI. II, fig. 14) of a small 

 blastozooid (4 mm. long) with M. IV-V laterally separate taken from the stolon of a solitary form in 

 which M. VIII-IX were parallel. 



