THE PHYLOGENY. OF THE. HIGHER CRUSTACEA, 99 
traca originated, but simply the nearest living ally of this ances- 
tral form. } 
He believes that the presence of a great mantle-like cara- 
pace, of eight unspecialized broad cormopods with leaf-like exo- 
podites, of a furcated abdomen without tail-fins, and of eight 
abdominal somites, show that it is not a Malacostracan, but a 
_phyllopod, As many phyllopods, such as Limnetis and the 
Cladocera, have, like the Malacostraca, an exopodite on the 
second antenna, we must believe that the Malacostraca have in- 
herited this feature from their phyllopod ancestor ; and, as it is 
absent in Nebalia, this form cannot be the direct ancestor of the 
Malacostraca. So, too, the fifth and sixth pairs of abdominal 
feet are rudimentary in Nebalia, while they are well developed 
in nearly all Malacostraca. As most of the phyllopods and 
some of the Malacostraca leave the egg as a free-swimming nau- 
plius, we must believe that this was true of the phyllopod ances- 
tor of the Malacostraca; but as Nebalia does not pass through a 
free nauplius stage, but leaves the egg in a more advanced con- 
dition, it cannot be in the direct line of evolution. Boas there- 
fore concludes that Nebalia is a true phyllopod, and that the 
Malacostraca have originated from a form somewhat different, 
although Nebalia is the closest living ally of this ancestral form. 
Having thus traced the decapods back through the Euphau- 
siacea to a phyllopod ancestor very similar to the recent Nebalia, 
we have now to trace the ancestry of the other Malacostraca. 
Boas holds that squilloids are a branch from the Euphausiacea, 
and that the Mysidacea have been derived from the Euphausia- 
cea along still another line of descent, and have in their turn, 
given rise to all the remaining groups of Malacostraca. 
The Mysidacea differ from the Euphausiacea and the deca- 
| pods in many features which they show in common with the 
Cumacea and the amphipods and isopods ; and it is not difficult 
| to show that, in these points of difference, the Euphausiacea are 
| the primitive group, and the Mysidacea the modified group. 
| In Euphausia, as in the swimming decapods, the body and 
| abdomen are compressed ; while they are flattened and rounded 
| in the Mysidacea, and the tip of the abdomen is directed back- 
wards, lacking the peculiar bend of Euphausia and Penaeus. 
The structure of the mandible is very instructive. In Mysis, 
_as well as in Cumacea and amphipods and isopods, the mandi- 
ble is forked, the cutting part being widely separated from the 
_ crushing part ; and between the two there is a row of setee, and 
| a peculiar accessary appendix. In Euphausia and the decapods 
| the appendix and row of sete are absent, and the chewing part 
| is hardly separated from the crushing part. In Mysis, as in 
| Cuma and the amphipods and isopods, the palp and exopodite 
| of the first maxilla are absent, and the laciniz are turned for- 
wards as well as inwards; and in all these forms the laciniz of 
ithe second maxilla are directed forwards. They overlap, and 
‘the lacinia interna is undivided. In Euphausia, the decapods, 
and squillas, there are no brood-pouches; but these structures 
a 
