336 JOURNAL OF CoMPARATIVE NEUROLOGY. 
segment becomes reduced and disappears. The next but one, 
which bears the antenne, is regarded by most writers as a meta- 
mere of post-oral origin. Concerning the remaining segment, 
there are three distinct views extant: I, it represents the pro- 
stomium of the annelids; 2, it is merely a specialized anterior 
region of the antennal segment, which has been caused by a 
secondary subdivision ; 3, it isa true metamere. The third 
view seems the most probable. It seems that, in the Hexa- 
poda, the prostomium and archicerebrum have not been plainly 
distinguished. 
In the Avachnida, where the head appears to be composed 
of two segments, the cheliceral and the procephalic, the cheli- 
ceral segment is undoubtedly a true metamere; but the pro- 
cephalic segment offers the same difficulties of interpretation as 
does the most anterior (cephalic) segment of the insect head. 
It is probably a metamere. 
In the Crustacea, where the head is composed of six seg- 
ments, the last three are obviously true metameres ; the next 
two are evidently post-oral metameres that have become pre- 
oral; as to the next there is much uncertainty. 
Although much evidence is needed to substantiate the 
view, yetit seems quite likely that, throughout the Arthropoda, 
the procephalic lobes of the head are homologous and represent 
the peristomial segment of the annelids. - 
The following table, which has been copied from the paper 
of Dr. Goopricu gives his views in a nutshell. 
Annelida Peripatoidea Insecta Arachnida Crustacea 
Prostomium with ty ? ? ? 
or without tentacles 
Archicerebrum Frontal processes Frontal processes 
Segment I or Peri-|Procephalic lobes,| Procephalic Procephalic Procephalic lobes, 
stomium antenne, proto-|lobes, protocer-|lobes, proto-|protocerebrum 
cerebrum ebrum cerebrum 
Segment II Mandibles, deuto-| Antenne, deu-|Chelicere, Ist antenne, deu- 
cerebrum tocerebrum deutocerebrum |tocerebrum 
Segment IIT Oral papilla Rudimentary |Trunk segment|/2nd antenne, tri- 
appendage, tri- tocerebrum 
tocerebrum 
Segment IV Trunk segment Mandibles Trunk segment} Mandibles 
Segment V Trunk segment 1st maxille Trunk segment|ist maxille 
Segment VI 
Trunk segment 
2nd maxille 
Trunk segment 
2nd maxille 
