146 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ment, because provided with a ganglion. More conclusive proof could 
hardly be expected. 
The insect-head, then, is composed of seven somites, which are homolo- 
gous with the first seven of decapod Crustacea. 
If the conclusions I have drawn in this paper are valid, certain radical 
changes become necessary in the commonly accepted ideas of homology 
among the great classes of Arthropods. These changes I submit in the 
following table : — 
TABLE OF EQUIVALENT SOMITES IN THE HEAD OF ARTHROPODA. 
Segment| Arachnida | Chilopoda Diplopoda Crustacea Hexapoda 
Compound 
eyes and 
ocelli 
Compound 
eyes and 
ocelli 
First 
antenne 
Embryonic 
preantenne. 
Antenne 
Second 
antenne 
Intercalary 
appendages 
Antenne Antenne 
Chelicerz 
Pedipalpi 
Mandibles 
First 
maxille 
Second 
Mandibles 
Mandibles 
First 
maxille 
Second 
Mandibles 
Superlingue 
Maxille 
First legs Gnathochilarium 
maxille maxille 
First Labium 
Second legs Maxillipedes 
maxillipedes 
Summary. 
The protocerebrum of Apterygota agrees with that of other insects 
in development and structure. The ocular segments of Hexapoda and 
decapod Crustacea, as well as the compound eyes of the two groups, are 
homologous. 
The labrum and clypeus of insects develop from a single median 
evagination between the procephalic lobes, and do not represent a pair 
of appendages. The labrum of Apterygota is homologous with that of 
other insects, as well as that of Symphyla, Diplopoda, Chilopoda, and 
the higher Crustacea. 
