76 ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 
three anterior thoracic somites, in A7Zyszs by the cephalic and six or 
seven anterior thoracic, and in ordinary Podophthalmia by all the 
cephalic and thoracic somites. 
g. Lastly, there are certain parts developed singly in the median 
line in the Aréevlata. Of this nature are the frontal spines of 
Crustacea, their telson, and the sting of the Scorpion, whose mode of 
development appears to be precisely similar to that of a telson. In 
the same category we must rank the labrum in front of the mouth, 
which in the Crustacea (at least) appears to be developed from the 
sternum of the antennary or third somite, the metastoma (or so-called 
Jabium or lingua) of Cyavstacea, and the lingua of /zsecta, behind the 
oral aperture. 
However much these appendages may occasionally simulate, or 
play the part of, appendages, it is important to remember that, 
morphologically, they are of a very different nature, and that the 
confusing them with true appendages must tend completely to 
obscure the beautiful relations which obtain among the different 
classes of the Articaulata. 
$5. The Embryogeny of the Articulata, Mollusca, and Vertebrata 
compared, 
[ find it difficult to conclude this memoir without saying a few 
words on the resemblances and differences between the embryogenic 
changes of the dr¢icvlata and those of the Wol/usca and Vertebrata. 
-\bsolute and fundamental differences appear to me to separate the 
members of these three classes almost from the first appearance of 
the germ. 
As we have seen, it is the neural side of the Arthropod which is 
first developed, while, so far as I am aware, it is the opposite or 
hamal side which is first formed in every Mollusk. The germ of the 
Arthropod becomes antero-posteriorly segmented; the germ of the 
Mollusk never does so. From these two fundamental differences a 
multitude of others necessarily follow. 
The Articulate embryo is no less markedly separated from that 
of a Vertebrate animal, although in the latter, as in the former, it is 
the neural surface which is first developed ; for I know of nothing in 
the Articulate embryo to be compared with the primitive groove, the 
chorda dorsalis, and the dorsal plates of the Vertebrate! They, like 
I therefore by no means agree with what Zaddach says on this subject, or with regard 
to the homologue of the amnion in Ardzcadata. 
