DISTRIBUTION OF THE CHAETOGNATHA 207 



This remarkable species was first described from eight specimens in the Gauss collections which were 

 taken in deep hauls (3423-2000 m.) from far south in the antarctic zone. Only one further specimen 

 has been reported, that found by the Russian expedition to the Kuriles trench which was taken between 

 5000 m. and the surface (Tchindonova, 1955). The specimen reported as //. mirabilis by Jameson(i9i4) 

 seems, from its head armature, to have been a damaged specimen of E. hamata. 



Two specimens have been found in Discovery collections (Station 661, 3000-2000 m.), but neither 

 of them is in good enough condition to determine the precise shape of the fins, and it seems worth 

 while considering whether these specimens could be distorted examples of some better-known form. 

 The presence of transverse musculature on the tail segment was regarded by Ritter-Zahony as a point 

 of great importance ('the tail has no homologue in any hitherto known chaetognaths '), but Tokioka 

 (1952, p. 312), referring to Zahonya cestoda van Oye (an enigmatic deep-living chaetognath), remarks: 

 ' It is a noteworthy fact that some species with strongly developed musculature assume an appearance 

 when they are in a strongly contracted state, as if they were provided with a transverse musculature 

 along the whole body.' This appearance can occasionally be observed in specimens of E. hamata. The 

 Discovery specimens of H. mirabilis do show transverse musculature in the tail, but they do not appear 

 to be at all strongly contracted. The number of anterior teeth in this species (see Table 2) is strikingly 

 large in comparison with other antarctic deep-water forms (15, as compared with 10 in 5. macrocephala, 

 and 8 in S. marri), and, as Ritter-Zahony has pointed out, only S. helenae (a tropical epiplanktonic 

 species) has comparable numbers. Ritter-Zahony records that the gut of the largest of his specimens 

 was brick red when fresh, and although the colour has faded from the Discovery- specimens the 

 appearance of the gut is the same as that in specimens of S. macrocephala and E. fowleri, both of 

 which have a red gut when fresh. 



Ritter-Zahony was unable to find any eyes in his specimens, and both Discovery specimens are also 

 apparently without eyes. Neither of the Discovery specimens has ripe seminal vesicles, but the smaller 

 animal has rudiments of them in the same position as those 

 figured by Ritter-Zahony in his fig. 46. From the tail- 

 segment percentage and general appearance the most likely 

 species which could be confused with H. mirabilis is 

 S. macrocephala, but the vestibular organ in these two 

 species is quite different, being quite smooth in H. mirabilis 

 (see Fig. 2) and distinctly papillated in S. macrocephala, and 

 although Ritter-Zahony stated that in young specimens of 

 H. mirabilis small papillae were present, they are present in 

 S. macrocephala at all sizes. Ritter-Zahony did not record 

 S. macrocephala from the Antarctic, yet it is not an uncommon 

 species in the Discovery collections and this would lead one 

 to suspect that H. mirabilis was indeed a damaged specimen of that species. Nevertheless, bearmg all 

 these possibilities in mind I am convinced that H. mirabilis is a valid species, though I feel that really 

 perfect specimens might show that it should belong to the genus Sagitta. 



Horizontal distribution. Antarctic, between 6^" 29' S — 65° 18' S and 80° 00' E 85° 27 E 

 (Gauss), Discovery Station 661, 57° 36' S 29° 54' W. North Pacific. 



Fig. 2. The head of H. mirabilis seen 

 from an antero-dorsal direction. 



III. Exotic species 

 This group is composed of seven species which will not be discussed in detail as they are all sub- 

 tropical or tropical forms which almost certainly do not maintain themselves in the Southern 

 Ocean. 



