. Bulletin of the British Museum (Natural History). 438 SAGITTA PLANCTONIS AND RELATED FORMS water which agree with Ritter-Zahony's table (1911, p. 30, " 90 specimens from the Atlantic Ocean"), but which have not been found with intact seminal vesicles. Traces of these structures however have been seen which appeared to be differently situated from those figured by Ritter-Zahony (1911, fig. 36). Moreover although the distribution of S. planctonis is well established as mesoplanktonic, specimens apparently of the same species ( within the range of variation given, or implied, by R
. Bulletin of the British Museum (Natural History). 438 SAGITTA PLANCTONIS AND RELATED FORMS water which agree with Ritter-Zahony's table (1911, p. 30, " 90 specimens from the Atlantic Ocean"), but which have not been found with intact seminal vesicles. Traces of these structures however have been seen which appeared to be differently situated from those figured by Ritter-Zahony (1911, fig. 36). Moreover although the distribution of S. planctonis is well established as mesoplanktonic, specimens apparently of the same species ( within the range of variation given, or implied, by Ritter-Zahony) have been taken in shallow hauls in the subtropics, associated with typically epiplanktonic forms, and often in too large numbers for this anomalous distribution to be ascribed to some freak of distribution of the mesoplankton. Thus, from distributional evidence it seemed that there were at least two separate species in the collections grouped under one name, an opinion held for many years by Mr. J. W. S. Marr (personal communication). In the absence of any established anatomical differences this opinion remained Fig. I.—Seminal vesicle of S. planctonis. Recently Mr. A. de C. Baker (also working on the "Discovery" collections) noticed, and drew my attention to, a large mature chaetognath in perfect condition (PI. la) which was taken at Stn. 1685 (41° 2i-8' S., 148° 51' E. ) to the East of Tasmania between 1000 and 750 m. Although the seminal vesicles (see Text-fig. i) of this specimen were not in agreement with Rittcr-Zahony's figure the specimen was in other respects identical with the subtropical forms referred to as S. planctonis. The Tasman sea is an area where S. planctonis is found in the surface waters as it is off Bermuda (Moore, 1949), Africa (Steinhaus, 1896), in the Aghulas current, off New Zealand and in parts of the North Atlantic (" Discovery" collections). I therefore supposed at first that thi
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