TurRNER, Mushroom Bodies of the Crayfish. 335 
ceeding segmental ganglia; and (3) also a nephridium or head 
kidney. In Polyche@ies the ccelom of the peristomium is often 
obscure. In some cases a pair of somites are formed in the 
peristomium, which, becoming hollowed out, may even give 
rise to peritoneal funnels. Inthe Polychetes nephridia are usu- 
ally developed. 
The prostomium presents none of the characters of a true 
segment. It neither surrounds the alimentary canal nor contains 
a pair of mesoblastic somites nor develops within itself neph- 
rida. Although finally becoming united with the ccelom, the 
cavity of the prostomium is primitively a blood-space and is 
coelomic only by virtue of its connections. From one or more 
centers situated in the upper surface of this prostomium is de- 
veloped the supra-oesophageal ganglion. 
This prostomium is neither a vestigial nor an incipient 
metamere ; it is either a remnant of the region lying in front 
of the mouth of the primitive unsegmented ancestor of the 
worms, or else an outgrowth from the peristomium. 
Since the region in front of the mouth in Arthropods is 
‘the fused product of several somites, the mouth has probably 
migrated backwards (caudad). 
In Pertpatus, where the head is composed of three seg- 
ments, all agree that the two posterior (caudal) ones are true 
metameres, but some doubt exists as to the nature of the pre- 
oral, eye-bearing, segment. The researches of voN KENNEL and 
SEDGWICK which show that in its development, this segment 
resembles a true metamere, and the researches of KORSCHELT 
and Herper, which indicate that the antenne of Pestpatus were 
originally post-oral, demonstrate that this first segment of Per- 
patus isa true metamere. The prostomium of Ferzpatus is in- 
significant ; the antenne arise from the first segment. 
The morphology of the myriopod head is imperfectly 
known, but it seems to resemble Peripatus. 
In the Hexapoda, where the head is composed of six re- 
gions, three posterior (caudal) regions, belonging to the labium, 
maxilla and mandibles, are universally considered true meta- 
meres. Counting from behind forwards, in the adult, the next 
