MORPHOLOGICAL CHARACTERS OF TAXONOMIC IMPORTANCE 23 



occur in the Southern Ocean, S. thompsoni and S. gerlachet, are typically more heavily serrated than 

 5.<7i'/)^a, having the arrangement of serrated ridges shown in PL II 6 and Text-fig. 2b. In the aggregate 

 form of S. aspera (PI. lb) the serrations appear to be irregularly scattered over the test with a tendency 

 to be well developed on the anterior and posterior processes. 



The solitary, like the aggregate, form of S. fusiformts has a smooth test with a pattern of thickened 

 ridges similar to that of S. aspera (PI. Ic), except of course for the absence of serrations on all ridges, 

 and is identical with the descriptions given by Herdman (1888) and Ritter (1905). A basically similar 

 arrangement of serrated ridges is found in S. thompsoni (PI. II a, c) and S. gerlachet (Text-fig. 2 a) except 

 for a more elaborate pattern on the dorsal surface where the secondary ridges, and particularly the 

 presence of a medio-dorsal ridge Cridge no. 9 in PI. lie), distinguish these two species from S. aspera. 

 It is thus possible, using the external character of the test of the solitary form, to distinguish between 

 S. fusiformts, S. aspera and S. thompsorti or S. gerlachei. Other characters, however, must be used to 

 distinguish between S. thompsoni and S. gerlachei. 



It has often been suggested that serrations are associated with age. Both Ritter (1905) and Farran 

 (1906), for example, conclude that in S. fusiformis aspera serrations occur in the larger, older speci- 

 mens. Apstein (18946), on the other hand, describes serrations in the smaller, younger specimens of 

 Thalia democratica. Sewell (1953), however, considers that 'such a condition of the test may occur 

 at any age up to mid-life, after which they tend to be eroded away'. In the present study it has been 

 possible to examine specimens of at least one serrated species, S. thompsoni, in every stage of its 

 development and so it is possible to make some generalizations about the occurrence of denticulations. 



Very young aggregates (3-4 mm. long) while still part of the stolon of the solitary form have a thin 

 poorly developed test which shows no external sign of serrations. The smallest released free-living 

 aggregates, 5 mm. in length, however, have a well-developed test characteristically serrated. It thus 

 seems reasonable to infer that when a block or chain of aggregates is released from the solitary form 

 there is very rapid test development which in S. thompsoni and S. gerlachei and probably also S. aspera 

 is accompanied by the formation of serrated ridges. In larger, older specimens as Sewell (1953) 

 described, the denticulations tend to wear away but in the largest specimens of the aggregate form 

 of each serrated species examined some serrations were quite obvious. 



The solitary form, while still an embryo within the oozoid (aggregate), has a poorly developed test 

 with no indication of denticulations, even though it may, before its release, attain a size of 9-10 mm., 

 having all the morphological features of the adult, including a rudimentary stolon, already developed 

 within it. In free-living solitary individuals in the 10-15 ^^- ^i^^ range the characteristic pattern of 

 serrated ridges can be seen, so that as with the aggregate form one can infer that serrations develop 

 very rapidly on the release of the embryo. Larger specimens often show evidence of a worn test, but 

 in all cases examined it was possible to see the pattern of serrations and so distinguish, for example, 

 S. aspera from S. thompsoni or 5. gerlachei. 



The Width of the Body Muscles in Salpa thompsoni and S. gerlachei 

 It will be realized from the previous sections that the only precise taxonomic difference between 

 S. thompsoni and S. gerlachei is in the number of fibres per muscle band. Other characters, however, 

 which are less easy to define in precise terms, make it possible readily to distinguish these two species. 

 One such character is the difference in relative width of the muscle bands in specimens of approxi- 

 mately the same size. 



Contained in the preliminary analysis of the samples collected by R.R.S. 'Discovery IF in the 

 Southern Ocean are reports of the occurrence of specimens of ' S. fusiformis aspera ' with ' narrow or 

 thin muscles ', such comments being restricted to those samples taken in high latitude Pacific waters. 



