SAGITTA GAZELLAE 243 



Stage II. The onset of stage II is sharply defined, and the complete filling of the tail segment with 

 sperm was chosen as the feature of this stage, partly because it is immediately visible to the naked eye, 

 and partly because its occurrence is so abrupt. Very few animals need be recorded as intermediate 

 (stage I/II). Animals at stage II (PI. XIII, fig. 1) can be picked out immediately by the complete 

 opacity of their tail segments. The ovaries continue to grow during this stage as do the seminal vesicles, 

 but the tail segment remains opaque. 



The growth of the animal takes place during stages I and II, and before reaching stage III it has 

 attained its full size. The relation between growth and the attainment of maturity is discussed on 

 p. 272. 



Stage III (PI. XIII, fig. 3). The transfer of sperm from the tail segment to the now fully developed 

 seminal vesicles marks the onset of this stage. The tail segment is once more translucent, and the 

 seminal vesicles, as seen in the living animal, are chalky white and very conspicuous. Although the 

 onset of this stage appears to be fairly sudden, several specimens have been taken which show an 



Table 5. A series of Subantarctic specimens of S. gazellae showing head armature and ovary 



size as a percentage of total body length 



intermediate condition (PI. XIII, fig. 2), and these have opaque tail segments with large opaque 

 vesicles; these have been recorded as stage II/III. Stage III ovaries are thin rod-like structures, 

 varying in length from 14 to 40% of the total body length. There is a reduction of the numbers of 

 posterior teeth at this stage ; these drop out in a haphazard manner from anywhere in the tooth row, 

 and not in a regular manner from one end, as is usually the case in the reduction of the hooks. The 

 loss of teeth coincides with the filling of the seminal vesicles and does not seem to be correlated with 

 the size of the ovaries. At this stage there is a reduction in hook numbers, and the base of the hooks 

 becomes slightly claw-shaped. Table 5 shows a series of Subantarctic specimens arranged according to 

 the length of the ovaries calculated as a percentage of the total body length, and it gives the head 

 armature, maturity stages and presence or absence of the vesicles. In this table animals in stages III 

 and IV are arranged in order of ovary length, but the other stages (II and II/III) have been arranged 

 according to the relative advancement of the male genital products. It will be noticed that the ovary 

 percentage of some stage II animals exceeds that of some stage III animals. 



