1896.] OF THE OBNUS SEROESTES. 945 



considered as the real Mastigopus, and even sometimes to determine 

 tlie AcaniJiosomci , consequently to elucidate at least one-half of the 

 total motamorpliosis. As a rule the diltcronces between the same 

 older stage of any two species whatsoever are more coQspicuous than 

 the differences between the species of Sergestes to which they 

 belong. During a long-continued study of a rich collection it will 

 gradually be possible to arrive at complete certainty in the 

 collocation of the series of stages of all species well represented, 

 but in too numerous instances it is impossible to refer the forms 

 to the representations of authors without examining their type- 

 specimens. 



The characters which undergo very little or almost no change 

 during the raetamorpiiosis from the older Mastigopus to the adult, 

 and for that reason offer good marks for identification, are the 

 following : — (1) the structure of mxp.^, viz., whether they are 

 scarcely longer than trl.' with the 4 proximal joints of the same 

 aspect (the fourth joint flattened) as in trl.^ and the two distal 

 joints equally setaceous on both margins — or whether they are 

 considerably or much longer than trl.', with the 4 proximal joints 

 considerably thickened and much more robust than in trl.", and 

 the 2 distal joints almost or totally naked along the one margin, 

 while at least the sixth joint is armed with several long and a 

 number of shorter spines on the other margin (but only the 

 presence of spines, not their number, can here be taken into 

 consideration; (2) the proportion between the naked and the hairy 

 part of the external margin of ext. br. of urp. ; (3) the relative 

 length of the 3 joints (especially the first and the third) of the 

 antenn. pad. ; (4) the number of subjoints in the sixth joint 

 of mxp.' — Other characters of more secondary value will be pointed 

 out in dealing with the species. 



The character derived from the length of mxp.', and especially 

 from the aspect of their 4 proximal joints in contradistinction to 

 the legs and especially to trl.'', can also he used in every Mastigopus- 

 stage ; it will even almost always be possible to refer a larva with 

 mxp.^ broken off to one of the two groups by comparing the basal 

 joint, which always persists, with the basal joints of the 3 

 following pairs of trunk-legs. — The character from the uropods is 

 in most cases more or less subject to alteration during the 

 development, and as a general rule it may be stated, that when only 

 g-| of the. exterior margin is hairy in the adult, then this part 

 approximates more and more towards occupying J of the margin 

 according to the youth of the specimens ; but when the margin is 

 hairy in the total or in c. ^ of its length in the adult, then the 

 hairy part is a little shorter in the younger, and still somewhat 

 shorter in the youngest Mastigopus. — The character from the 

 length of the joints in antenn. ped. also alters in the younger 

 stages, with the result that the first joint is proportionally longer 

 (and distally much narrower) in the younger than in the older 

 Mastigopus-stagns. — Several instances proving these rules will be 

 found in the following descriptions of the species.. 



61* 



