MORPHOLOGICAL CHARACTERS OF TAXONOMIC IMPORTANCE 27 



In S. thotupsom the first fully differentiated block of buds occurs in the 30-40 mm. size range and 

 may be released, as indicated by the presence of a strand, at 50-60 mm. although release may not 

 occur until the animal is 70-80 mm. long. The number of buds in block i is remarkably constant in 

 each size group with a mean, based on 205 specimens, of 134-8 buds. It will be noted that subsequent 

 blocks, as shown by the data from specimens with strands (block 2, in the figure) have a significantly 

 greater number of buds than block i , which suggests that the animal's capacity to produce buds increases 

 as it grows. This view is further supported by the very high mean values of 297-2 and 262-0 buds per 

 block for animals in the 90-100 mm. and loo-i 10 mm. size range; these blocks, as mentioned above, 

 probably being blocks 3 or 4 — in other words those produced subsequently to the release of block 2. 

 In S. gerlachei, block i is fully segmented at a body size of 20-30 mm. compared with 30-40 mm. 

 in 5. thompsoni, while it is released at a size of 30-40 mm. compared with 50-60 mm. in the other 

 species. Block i in S. gerlachei has strikingly fewer buds than 5. thompsoni with an average of 38-0 buds 

 per block, although it should be noted that this is based on only six specimens. Subsequent blocks 

 have about the same mean number of buds as in specimens of S. thompsoni of comparable body length. 

 The data thus show that compared with S. thompsoni, S. gerlachei produces initially in block i 

 significantly fewer buds which are released when the solitary form is relatively much smaller. These 

 differences in asexual development might be attributed to environmental change, the specimens being 

 geographical races of the same species. In view, however, of the differences in morphological character 

 that are apparent in both aggregate and solitary forms it is assumed that they are specific differences. 



DISTRIBUTION 



Any consideration of the distribution of the four species described in this paper is limited by the 

 geographical range of the stations from which samples were available for study. As mentioned 

 previously (p. 5) most of the stations are from the Southern Ocean and only a relatively few lines 

 of observations extend north of the subtropical convergence. These latter stations have been supple- 

 mented by material made available by other workers which, however, as a consequence are rather 

 scattered in geographical location. Nevertheless, in spite of the relative paucity of data from areas 

 other than the Southern Ocean, an attempt has been made in Text-fig. 10 to show the occurrence of the 

 four species on a world chart using all the stations from which material has been personally examined. 



Text-fig. 10 shows the positions at which the various species have been recorded, each species being 

 indicated by a different symbol. Also indicated in the figure is the mean position of the subtropical 

 convergence based on Deacon (1937). With regard to this convergence it should be remembered that 

 although as a physical-chemical feature it is well defined it is more variable in position than the 

 antarctic convergence ; and, furthermore, its position in some areas, particularly the eastern Pacific, 

 is largely conjectural. 



From Text-fig. 10 it is obvious that the region of the subtropical convergence represents a faunistic 

 boundary, at which two of the species, S. fusiformis and S. aspera, reach the southern limit of their 

 distribution. For S. thompsoni it is the northern limit to its range. Records of these species either north 

 or south of what is apparently their normal range do occur, but in Text-fig. 10 the mean position of 

 the subtropical convergence is shown and if its contemporary position could have been plotted for 

 each crossing by a line of stations its appearance as a boundary would be even more clearly defined. 



Text-fig. 10 shows that both S. fusiformis and 5. aspera have been taken north of the subtropical 

 convergence in Pacific, Atlantic and Indian Oceans. It is not possible from th^ data available to define 

 the northern limit to the distribution of either species, ahhough in the present material, S. fusiformis 

 has been recorded as far north as 63° 5 1 ' N., 16° 20' W. while S. aspera has been recorded at 37° 24' N. 



4-2 



