REPORT ON THE CRUSTACEA MACRURA. 471 



in a rudimentary condition, and the pereiopoda are in their entirety utilised as natatory 

 organs, it is doubtful if the basecphyses be ever used for simply swimming purposes; and, 

 moreover, I think it to be capable of demonstration that in the pelagic Macrura, when 

 these organs are developed, they are mostly employed for ascending and descending 

 through the various strata of oceanic water, and are of importance in enabling the 

 animals to avoid the strong sunlight at the surface, by permitting them to sink during 

 the daytime, and ascend at night, a periodic movement that has been so frecpiently 

 noticed to be their habit. 



2. The large size of the synaphipod attached to the mandibles in many of this tribe 

 is a feature the animals possess in somewhat inferior degree to that which we see in 

 Lucifer, Sergestes, and other genera of the Penseidea, more especially in Gennadas and 

 Benthesicymus, where they frequently reach to the extremity of the ophthalmopoda, but 

 the size of this appendage cannot be considered as being a condition illustrative of the 

 tribe, inasmuch as it is absent in the genus Stylocheiron, just as we find it wanting in 

 some genera of the Phyllobranchiata. 



3. The pereiopoda are very variable in form and proportion among the several 

 families of which this tribe is composed, when compared with the more normal 

 forms ; and the gnathopoda exhibit a persistent tendency to approach to the pediform 

 condition of the pereiopoda, thus illustrating a very constant law that, with the 

 depreciation of the functional power in the pereiopoda, the gnathopoda increase in 

 importance. This appears to me to be only a continuation of the same process that is 

 visible all through the Macrura, and which is strongly exhibited in the Sergestidae, and 

 becomes more exaggerated in the Sehizopoda — leading through the Eucopiidae, in which 

 we find the pereiopoda departing from the normal form in this division of the Crustacea, 

 and approximating to those of the Amphipoda, in which the first pair of pereiopoda and 

 both pairs of gnathopoda are subchelate. To carry the similitude further, all the somites 

 of the pereion in Eucopia complete their dorsal arc as in the Amphipoda ; the carapace 

 being only a thin and membranous cloak that loosely overlaps the pereion. 1 



4. The mode by which the ova are carried by the females in the Sehizopoda varies. 

 In the Euphausiidse it corresponds with that of Lucifer, and may be the same in the 

 Penaeidse also, but I am inclined to believe that the weight of evidence is in favour of the 

 belief that the ova in the normal group are deposited in the open waters of the ocean. In 

 other families of the Sehizopoda they are supported in pouches of different kinds, developed 

 for the purpose beneath the pereion, similar in kind but varying in different genera, 

 like those of the Edriophthalmous Crustacea, or they are carried in sacs as in Entomos- 

 tracous forms. In the Euphausiidse, moreover, the males possess a petasma attached to 



1 Apsendes talpa, Montagu, appears to approach nearer to this tribe than to the Normal Isopoda. It only- 

 wants the extension of the cephalon into the form of a carapace to give it all the characters essential to a perfect 

 Macruran. 



