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Show 1896.] OF THE GENUS SERGESTES. 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 hauptslichlicbe Unterschied von der erwachs-enen Sergestes-¥oYm,,, which in this draught is rather obscure, and this author has also accepted the larvae described by his predecessors as adults, as being valid species of Sergestes. iii. The adult. Sergestes and Mastigopus. No author bas 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 the exterior flagellum of the antennulse. 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 determined, are comparatively large, at least 15-25 m m . in length and sometimes much longer, that they all possess short eye-stalks with rather small or very small and totally black eyes, and that they have the fifth pair of trunk-legs tolerably developed and the 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 antennulse have been found, and all these species are rather small, rarely more than 4-15 m m . long, almost all with rather long or long eye-stalks, rather large or large eyes, all with the eyes either totally yellowish (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. atlantlcus,M. Edw.,_with black eyes, not rarely, were found somewhat smaller specimens with pale or |
