SAGITTA GAZELLAE 241 



fixation. The same specimens were measured in 1952 and 1954, and although the number of observa- 

 tions is very small, the results are fairly consistent, and suggest that, disregarding the rather large 

 difference which occurs between live and fixed animals, most of the shrinkage occurs during the first 

 two years in preservative (Table 3). It is quite possible that anomalies in tables of head armature and 

 tail percentages may be due to shrinkage, as the tail segment is often much less affected than the rest of 

 the body. This is especially so in animals at stage II (those with their tail segments packed with sperms). 

 Bollmann (1934) remarks that preservation affects the appearance of the hooks in the closely allied 

 species S. lyra, but no difference, other than a slight increase in opacity, has been observed between 

 the hooks of live and dead specimens of S. gazellae. 



Table 3. Shrinkage of a series of specimens shortly after preservation and after 



several years in formalin 



(Each column of figures refers to one specimen.) 



Hooks and teeth (in animals over about 12 mm. long) have been examined and counted under a 

 low-power binocular microscope ; for animals below this length a monocular has been used. The head 

 armature can best be seen if the animal is held by the pressure of a mounted needle just behind the 

 corona; the anterior part of the head then faces the objective of the microscope, and by varying the 

 pressure on the mounted needle the whole armature can be seen, and counted quite rapidly. Occa- 

 sionally the use of methylene blue has helped in counting teeth, as it leaves an area free of stain at the 

 base of each tooth. 



All counts of hooks include the rudimentary ones. These are often very small, and may be only just 

 projecting through the cuticle. They are only present in animals below about 30 mm. in length, and are 

 usually easily seen, if strong reflected light is used ; they are sometimes difficult to see by transmitted light. 



A weak aqueous solution of methylene blue has been used to show the corona. As Thomson (1947) 

 has observed, methylene blue fades rapidly, and animals stained with it can be returned to samples 

 after examination. For permanent preparations a dilute solution of aqueous haematoxylin has proved 

 fairly satisfactory, though it does not show the corona and sensory spots as distinctly as methylene blue. 



The heads of some specimens have been boiled in caustic potash to verify the hook and teeth 

 numbers counted under the binocular microscope. There were no discrepancies, but it is possible that 

 among the large numbers of teeth counted, some immature ones may have been missed. It is unlikely, 

 however, that the hook numbers are in error, as even the rudimentary ones are large in comparison 

 with the smallest teeth. 



STAGES OF MATURITY 



Various systems for the classification of the stages of maturity of Chaetognaths have been proposed 

 by Russell (1932), Kramp (1939), Thomson (1947) and Pierce (1951). Although the Chaetognatha 

 are hermaphrodite some of these authors have based these stages only on the ovaries, but Kramp has 

 used both testes and ovaries, and this I think provides a sounder basis; though no system of this kind 

 seems universally applicable. Each species or closely related group of species tends to differ in the 



