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 betore 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 
of metameric 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 coudition which used to be passed through within 
the ege-membranes, as in WVebalia and the Mysidacea, when the 
order of differentiation of the segments was normal. ‘The case is 
a little different with the Nauphus 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 Euphausidae, in contradistinction to the Mysidae, are 
frequently met with in the surface-plankton. Huphausia 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 Euphausid organisation are 
brought out in Fig. 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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