944 DK. II. J. HANSEN ON CnUSTAOEANS [Dec. 1 , 



or furnishing with seta3 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 branchiDe or lamellse 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 

 elenderness of the body. Especially mxp? offers most valuaUe and 

 very neyleded dfferences. (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 Alastigopus-ioTms, 

 characters from all the structural features mentioned to be used in 

 the adults can be derived, aud 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 Seryestes and the collocation 

 of all the different stages of the same 7l/rtsii(/opMS-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, malciny use of 

 characters luMcli alter very little dtiring the older Mastigopus-staf/c* 

 and the development to the adtdt shape ; then he must search in the 

 collection for the oldest Mastiy opus- ST^ecxmens 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 circum- 

 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 fiipall " 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 develojimont 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 



