1896.] OF THE GENUS SBRGBSTBS. 941 



between Mastigopus and Sergestes have not been apprehended by 

 Bate. — In 1893 A. Ortmann (in his above mentioned paper) gives 

 a general view of the development of Sergestes ; on p. 68 he says 

 that the reduction of the two posterior pairs of trunk-legs in 

 Mastigopus "ist der hauptsachliche Unterscliied von der erwachs- 

 enen Sergestes-Form," which in this draught is rather obscure, and 

 this author has also accepted the larvae described by his prede- 

 cessors as adults, as being valid species of Sergestes. 



iii. TJie adult Sergestes and Mastigopus. 



No author has put or answered the question how to decide 

 whether a specimen of a Sergestes is really adult. At first sight 

 this does not seem to be the case. Long ago Milne-Edwards 

 discovered an organ only found in the adult (or subadult) male, 

 viz. a large and very complicated appendix on the first pair of 

 pleopods, the so-called " petasma," and Kroyer added the peculiar 

 development of Ihe exterior flagellum of the antennuloj. Later 

 on Bate, Smith, Wood-Mason, and Faxon have found similar 

 structures in some species. But it is interesting to observe that 

 all the species in which these structures have been found, or, in 

 other words, the species of which the male sex has been deter- 

 mined, are comparatively large, at least 15-25 mm. in length and 

 sometimes much longer, that they all possess short eye- stalks with 

 rather small or very small and totally Mack eyes, and that they have 

 the fifth pair of trunk-legs tolerably developed and tlie fourth pair 

 rather long and fringed with numerous long cilia ; while in most of 

 the described species no petasma and no transformation of the 

 exterior flagellum of the anfennulsB have been found, and all these 

 species are rather small, rarely more than 4-15 mm. long, almost 

 all with rather long or long eye-stalks, rather large or large eyes, 

 all with the eyes either totalli/ yellmvish (or whitish) or at most with 

 a blackish spot in the interior, and the fourth and especially the fifth 

 pair of trunk-legs rather short or even rudimentary. When 

 Kroyer published his monograph the development was quite 

 unknown, and not being able to find any male specimen of 

 numerous species he believed that his specimens were females. 

 Bate and Ortmann, who later on studied collections many times 

 richer than that examined by Kroyer, do not mention having 

 met with any male of any of the numerous smaller species ! 

 These results suggest that the smaller species must offer some 

 peculiarity. 



The collection of Sergestes in the Zoological Museum of the 

 University in Copenhagen is very large, 300 bottles and tubes 

 (each containing all the specimens of a species from the same 

 locality) ; all the animals, with extremely few exceptions, have been 

 collected with surface-nets. Trying to discriminate and determine 

 the forms, I soon took notice of the fact that among an enormous 

 material (98 tubes) of S. atlanticiis, M. Edw., with black eyes, not 

 rarely were found somewhat smaller specimens with pale or 



