944 DR. H. J. WANSEN ON CRUSTACEANS [ Dec. 1, 
or furnishing with setz or spines of the two distal joints, and the 
division of the sixth joint into 4, 5, 6, or 8 subjoints, &c.), the 
number and size of the branchiz or lamelle above trl.’ and trl.’, the 
difference in shape and the furnishing with cilia along the exterior 
margin of ext. br. of urp., finally sometimes the coarseness or 
slenderness of the body. specially map. offers most valuable and 
very neglected differences. (Of course it will also be possible to 
detect good characters in other parts, f. inst., in the structure of the 
5 pairs of trunk-legs, and one difference is used in the following 
discussion ; the petasma also exhibits characters, but this curious 
organ it is impossible to describe and make use of without figures.) 
It will, for the rest, be necessary to examine the animals much more 
scrupulously than has hitherto been done by most authors, for 
some described species are not recognizable, and at least S. edwardsi, 
Kr., is collective to such a degree, that between the limits adopted 
by W. Faxon it includes at least 4 species. 
For the discrimination and description of the Mastigopus-forms, 
characters from all the structural features mentioned to be used in 
the adults can be derived, and moreover the armature of the 
abdominal segments and the shape of the telson frequently offer 
good characters. But it must be remembered that alterations in 
almost all parts take place during the development from the 
youngest to the oldest larval stage, some of the alterations being 
very great, others rather small. To succeed in the double aim—the 
reference of the Mastigopus to the adult Sergestes and the collocation 
of all the different stages of the same Mastigopus-species, distin- 
guishing them from the stages of other species—we have but one 
way to go, which, in reality, is rather troublesome. (The deve- 
lopment in aquaria of the various stages may be possible, but almost 
’ all species being tropical or subtropical, and besides belonging to 
the open sea, very little help from this method can be expected for 
many years.) The student must work with copious material, and 
having isolated and examined and determined all the specimens with 
black eyes, he must subdivide the species into groups, making use of 
characters which alter very little during the older Mastigopus-stages 
and the development to the adult shape ; then he must search in the 
collection for the oldest Mastigopus-specimens which coincide with 
the adults in the characters mentioned, and try to refer them to 
the adults; at last he, being especially assisted by most of the same 
characters, must try to proceed backwards from the older to the 
younger and then to the youngest stages of every species, wherein 
he will in numerous instances be much assisted by the cireum- 
stance that different stages of the same Mastigopus are frequently 
taken together in the same haul. (Some authors not infrequently 
write in the descriptions of the small “ species” that the specimens 
vary in several particulars, f. inst. in the development of the 
dorsal abdominal spines, and this is often derived from the fact 
that their degree of development has been somewhat unequal.) 
Applying this principle it will in many instances be possible to 
determine the youngest forms, which by Bate and Ortmann are 
