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 hauptsiichliche Unterschied von der erwachs- 
enen Sergestes-Form,” which in this draught is rather obscure, and 
this author has also accepted the larve described by his prede- 
cessors as adults, as being valid species of Sergestes. 
ii. The 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 the exterior flagellum of the antennule. 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 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 antennule 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 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 
Kréyer 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. atlanticus, M. Edw., with black eyes, not 
rarely were found somewhat smaller specimens with pale or 
