DESCRIPTION OF THE SPECIES 15 



Alimentary canal. Compact, oval nucleus. 

 Embryo. Single as in S. thompsoni. 



Distribution of 5. gerlachei. Confined to the high latitudes of the Pacific Sector of the 

 Southern Ocean. Not circumpolar in its distribution. 



KEY TO THE SPECIES 



(The diagnostic characters are summarized in Table i) 



Aggregate 



1. Test smooth; total fibre number 40-61 (mean 51-8). S. fusiformis 

 Test serrated; total fibre number more than 113. 2 



2. Muscles IV and V approach but not in contact laterally. S. aspera 

 Muscles IV and V in contact laterally. -, 



3. Muscle bands broad; total fibre number 140-235 (mean 1777) (ratio of body length:muscle width, 18-9: i). 



S. thompsoni 

 Muscle bands narrow; total fibre number 1 13-159 (mean 128-8) (ratio of body length : muscle width 27-6:1). 



S. gerlachei 



Solitary 



1. Test smooth; muscle fibres of M. IV 19-40 (mean 29-4). S. fusiformis 

 Test serrated; muscle fibres of M. IV 36-130. 2 



2. Muscle bands VIII and IX parallel (or nearly so); test ridges 'simple' (see p. 10). S. aspera 

 Muscle band VIII and IX joined; test ridges 'complex' (see p. 12). 3 



3. Muscle bands broad; fibres of M. IV 70-130 (mean 93-2) (ratio of body length : muscle width, 21-2: i). 



iS. thompsoni 

 Muscle bands narrow; fibres of M. IV 36-71 (mean 48-8) (ratio of body length : muscle width 35-8: i). 



S. gerlachei 



MORPHOLOGICAL CHARACTERS OF TAXONOMIC IMPORTANCE 



It is proposed in this section to deal with those aspects of the morphology that have proved to be of 

 importance in understanding the taxonomy of S. fusiformis and its related species. For a detailed 

 account of the morphology of the Salpidae in general reference should be made to the monographs 

 of Ihle (1937-58). In the present study variations in the structure and arrangement of tw^o features, 

 the external test and the body musculature, have been found useful in distinguishing the different 

 species, but it is not suggested that these same characters will necessarily be of equal significance 

 in the other genera. 



The Number of Fibres per Muscle Band 

 The muscle bands so characteristic of the Salpidae are composed of muscle fibres arranged parallel 

 to each other and to the main axis of the muscle, and in many species the number per muscle can be 

 counted using a low-power binocular microscope. Little significance has been attached to muscle- 

 fibre number and only a few workers have considered it to be of taxonomic importance. Apstein (1906) 

 distinguished between the aggregate forms of S. mucronata {= T. detnocratica) and S. flagellifera 

 {= T. longicauda) on the basis of the number of fibres per muscle which was found to be charac- 

 teristically different in the two species. Tokioka (1937) described from Japanese waters Thalia demo- 

 cratica var. orientalis using the form of the test and the high number of fibres per muscle to distinguish 

 it from the solitary form of T. detnocratica. Sewell (1953) discusses T. democratica and its related 

 forms in some detail and gives significantly different fibre counts for the type and for T. democratica 



