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Show 944 DR. H. J. HANSEN ON CRUSTACEANS [Dec. 1, or furnishing with setae or spines of the two distal joints, and division of the sixth joint into 4, 5, 6, or 8 subjoints, & c ) , the number and size of tbe branchiae or lamellae above trl.3 and trl.4, 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. Especially mxp? 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. edwardsl, Kr., is collective to such a degree, that between the limits adopted by W . Faxon it includes at least 4 species. For tbe discrimination and description of the Mastlgopws-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 Mastlgopus-s\>ec\es, distinguishing them from the stages of other species-we have but one way to go, which, in reality, is rather troublesome. (The development 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.) Tbe 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 Utile during the older Mastigopus-sta^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 circumstance 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 specimen's 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 |