A STUDY m MORPHOLOGY. 
113 
together, as rudimentary buds, before the exopodites of the pereiopods and maxillipeds 
disappear. 
The third column of Table VI. shows the resemblances to Lucifer and Acetes at 
this stage. 
In the immature or Mastigopus stage (see Claus’s c Ueber einige Schizopoden und 
niedere Malacostraken, Messinas ’ ) the three forms are almost exactly alike, except 
as far as the generic distinctions are concerned, and the young Sergestes scarcely 
differs from the young Lucifer except in the absence of a neck, the length of the 
flagellum of the second antenna, and the presence of rudiments of the fourth and fifth 
pairs of pereiopods. 
Comparing the whole course of development of the three forms, as far as it is known, 
we notice that while the larval stages of Sergestes are much more different than those 
of Acetes from the corresponding stages of Lucifer , the character of the. change at 
each moult is much more like what we have in Lucifer than what we have in Acetes. 
We cannot fail to notice, in the second place, that the attempt to express the facts 
of the metamorphosis of these forms, so far as we know them, in a tree-like diagram, 
would result in a tree placed upside down, with the branches which represent the 
three Protozoeas much more divergent than those which represent the three young 
Sergestids. A similarity of type runs through the whole metamorphosis, but it is 
no more marked at the early stages than it is in the late stages, while the secondary 
differences are much more conspicuous during the Zoea and Acanthosoma stages 
than they are as we approach the adult form. 
While this is true it is also true that if we imagine a metamorphosis which 
shall agree with these three in all their common features, but shall have none of the 
features which they do not all share, we shall have something much more like the 
metamorphosis of Lucifer than that of Acetes or Sergestes, and we must therefore 
regard the life-histories of these three forms as somewhat divergent modifications of a 
form of development which is at present more closely adhered to by Lucifer than by 
the other two, and in this metamorphosis we must recognise a Protozoeci stage when 
the two pairs of antennae are locomotor, the ocellus present, the labrum furnished with 
a spine, the carapace armed with posterior dorsal and lateral spines and a rostrum; 
the two pairs of maxillae, and two pairs of maxillipeds present, and the thoracic and 
abdominal segments without appendages. This stage persists, with slight modifi¬ 
cation, through several moults in all of them, and is followed by an Acanthosoma stage, 
in which the carapace has a rostrum and antero-lateral spines, and a smooth posterior 
edge; the eyes are stalked; the two pairs of antennm have their adult character ; 
there are at least four pairs of pereiopods with swimming exopodites; the swimmerets 
are large and have their adult form, and the other abdominal appendages are absent. 
I he duration of this stage and the mode of transition to the next varies in the three 
forms, but it is followed in all by what may be called a Mastigopus stage, characterised 
by the general features of the family, 
MDCCCLXXXII. q 
