CHAP. VI EUPHAUSIACEA — LARVAL HISTORY 145 



and the rudiments of the sixth pair of pleopods are already 

 visible. 



In the next stage (" Furcilia ") the other abdominal pleopods 

 are added, the whole series being completed before the thoracic 

 appendages number more than two or three. This stage 

 corresponds to the Metazoaea of the Decapoda, and the inter- 

 ference in the orderly differentiation of the segments with their 

 appendages from before backwards is a phenomenon which we 

 shall meet again when we treat of Decapod metamorphosis. It 

 is evidently a secondary modification, furnishing the larva preco- 

 ciously with its most important swimming organs so as to enable 

 it to lead a pelagic existence. The frequent violation of the law 

 ofmetameric segmentation, that the most anterior segments being 

 the first formed should be the first to be fully differentiated, leads 

 us to suppose that the larval stages of the Eucarida at any rate 

 do not represent phylogenetic adult stages through which the 

 Malacostraca have passed. Nor do they, perhaps, even represent 

 primitive larval stages, but have been secondarily acquired from 

 an embryonic condition which used to be passed through within 

 the egg-membranes, as in Nebalia and the Mysidacea, when the 

 order of differentiation of the segments was normal. The case is 

 a little different with the Nauplius larva. This larval form, in 

 an identical condition, is found both in the Entomostraca as a 

 general rule, and again in certain Malacostraca, viz. the Euphau- 

 siidae and the Peneidea. Whatever its phylogenetic meaning may 

 be, we may be quite certain that the ancestor of the two great 

 divisions of the Crustacea had a free-swimming Nauplius larva, 

 and this conclusion is confirmed by the probable presence of a 

 Nauplius larva in Trilobites. 



The Euphausiidae, in contradistinction to the Mysidae, are 

 frequently met with in the surface-plankton. Eufhausia pel- 

 lucida (Fig. 102) is of universal distribution, and is frequently 

 taken at the surface as well as at considerable depths. 



Many noteworthy features in Euphausiid organisation are 

 brought out in Eig. 102. The shrimp-like appearance of the 

 carapace and antennae indicate the special Decapodan affinities of 

 the family ; noteworthy, also, are the single series of gills and the 

 biramous thoracic and abdominal limbs, similar to those of the 

 Mysidacea. The Euphausiidae also possess phosphorescent 

 organs of a highly developed kind, and these are usually situated, 



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