MATERIAL AND METHODS 5 



ever, which were originally examined alive, or just a few minutes after preservation in 1951, showed 

 no significant change in size when re-examined in 1959. It thus seems safe to assume that the effects 

 of shrinkage due to preservation, at least in so far as they affect the morphological characters used in 

 this study, are negligible. 



The number of aggregate and solitary individuals of each species in each sample was counted while 

 the size composition of each form was determined by length measurements. The stage of development 

 of the embryo or stolon was assessed by detailed examination of the oozoid ( = solitary) and blasto- 

 zooid (= aggregate) with a low-power stereomicroscope. These measurements and counts result in 

 data that will form the basis of a later paper on life-history and distribution. The present work is 

 concerned with detailed structure in so far as it relates to the taxonomy of the animals, and this depends 

 on the detailed examination and measurements to be described. 



In many previous works devoted to the description of collections of salps measurements of length 

 are given often without the points of reference from which the measurements were made. This seems 

 to be of importance, because while some authors distinguish between measurements that include the 

 prolongations that often characterize the test, others do not, even though such extensions of the test 

 may represent a large percentage of the total length. While strictly speaking the length of a specimen 

 should be the distance between its anterior and posterior extremities such a measurement seems 

 unsatisfactory when applied to the Salpidae. Firstly, the test is very variable in form not only between 

 species but within species on account of age, natural wear and damage in the net, and, secondly, the 

 test frequently becomes separated from the body n very ri h samples presumably through damage 

 in the net, since salps are on the whole fragile animals. It thus seems more realistic to use measure- 

 ments of length based on the body (or mantle; see Yount, 1954) rather than the test. 



In the present study the following standard measurements are used : 



total length, the distance between the anterior and posterior extremities of the test; 



body lefjgth, the distance between the mouth and the atrial opening; 



nucleus length, the nucleus (or alimentary canal) is typically ovoid in the species to be considered 

 and so the measurement taken is the length of the major axis. 



Throughout this paper all lengths quoted will refer to body length unless otherwise stated. It will 

 be realized from the relative shape of the solitary and aggregate stages that in the former total length 

 and body length approximate to each other while in the latter they may be very different. Lengths 

 in all cases were measured to the nearest millimetre on a clear plastic graduated scale. With larger 

 specimens, particularly of the solitary form greater than 50-60 mm. in length, the measurement is 

 not assumed to be more accurate than ±2 mm., owing to the extreme elasticity of the body especially 

 in specimens that have become separated from their tests. However, this in no way invalidates con- 

 clusions drawn from such data which are always grouped for purposes of comparison into 5 or 10 mm. 

 size groups. 



The detailed examination of specimens was assisted by the use of aqueous stains. Toluidine blue 

 (Yount, 1954) or methylene blue in dilute solution allowed the surface of the test (p. 22) to be seen 

 in detail, and in some cases made visible denticulations on old worn tests that might well have been 

 classed as smooth. Rose Bengale (Yount, 1954) injected either through the mouth or the atrial 

 aperture into the body proved a useful method of seeing the arrangement of the body musculature, 

 particularly in small specimens. 



A low-power stereomicroscope proved adequate to see the structure of individual muscle bands and 

 except in very small specimens it was possible to count the number of constituent fibres in each 

 muscle (p. 1 5). Where necessary the widths of individual muscle bands (see p. 23) were measured with 

 a graduated eyepiece. 



