to receive the strong muscles moving these appendages. For this reason the legs 

 in the adult males often appear very unlike those in the females. 



The pleopoda are always wanting in the female, and sometimes in the 

 male also; but more generally the adult male possesses them either on all the 

 caudal segments except the last, or at least on some of the anterior segments. 

 These limbs are rather small, with the rami very short, and never composed of 

 more than 2 joints, and carrying at the tip long ciliated setse. In some cases 

 the rami are coalesced or quite rudimentary. 



The uropoda generally have the basal part very slender and elongated, 

 often much longer than the rami. The latter are narrow, styliform, and the in- 

 ner one often densely spinulous inside. 



The telson is only fully developed in 3 of the 8 families, viz., the 

 Lampropidce, Platyaspidce and Diastylidce. In the Pseudocumidce it is certainly 

 present, but only as a rudiment. 



The sexual dimorphism of the Cumacea is very pronounced, the adult 

 males looking, as a rule, very different from the females, both as regards their 

 outward appearance and the structure of the several appendages, whereas the 

 young, not yet sexually developed males on the whole closely resemble the females. 

 This circumstance has caused much confusion, and has given rise to the estab- 

 lishment of several spurious genera, even in quite recent times. 



Of the inner organisation and development of the Cumacea, a detailed 

 account will be given at the close of this work. I will here only remark 

 that the structure of the various internal organs, which is said to resemble closely 

 that in the Isopoda, agrees fully as well with that found in the lower Schi- 

 zopoda (Mysidse). The embryonal development also exhibits many points of 

 agreement with that in the Mysidce and Lopliogastridce, though the resemblance 

 with that of the Isopoda is perhaps still more obvious. The embryos undergo 

 their metamorphosis within a roomy marsupial pouch formed by 4 pairs of thin 

 plates issuing from the bases of the 3rd pair of maxillipeds and the 3 anterior 

 pairs of legs. On escaping from the marsupium, the young, like those of the 

 Isopoda, still want the last pair of legs. 



In occurrence, the Cumacea are exclusively marine, and, on the whole, true 

 bottom-forms, though the more agile adult males of some species may be found at 

 times swarming near the surface, especially at night. Some of the forms are 

 littoral or sub-littoral in their occurrence ; but by far the greater number of species 

 are pronounced deep-water forms, descending to the greatest depths explored. 

 Cumacea are found in every part of the ocean, and as far north as deep- 

 water exploration has been instituted, these peculiar Crustacea have been met 



