232 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 
ages and the general uniformity in plan, so far as the anterior part of the body is con. 
cerned, in all Arachnida. But it may be asked, what becomes of the hinder thoracic and 
the abdominal somites in the Spiders and Mites? Without, at present, giving a positive 
answer to this question, I am inclined to think that the Spiders stand to the Scorpion in 
the relation of Lemodipoda to Amphipoda, and that many of their posterior somites are 
aborted. 
I do not doubt that many minor variations will be detected when the morphology 
of the Articulata is carefully examined; but I venture to think it a result of no small 
moment, if it can be proved that a Lobster, a Cockroach, and a Scorpion are composed 
of the same primitive number of somites; that the head in each consists of the same 
number of parts, and that the great differences are the consequence of the different modi- 
fication of the thoracico-abdominal somites, all fourteen of which bear appendages in 
the Lobster, while only three (or if we consider the genital apparatus in the light of 
appendages, five) are so provided in the Insect, and only two (leaving out of consideration 
the “ pectines”) in the Scorpion. 
8. I have elsewhere* explained at length my views with regard to the nature of the 
carapace in the Crustacea, and I will only repeat here, that there seems to me to be no 
constancy in its composition. The rudimentary carapace of Squilla is assuredly deye- 
loped from not more than four somites, the antennary, mandibular, and maxillary. In 
Apus, I doubt whether more than the six cephalic somites enter into its composition, 
In Cwma it is constituted by the cephalic and three anterior thoracic somites, in Mysis 
by the cephalic and six or seven anterior thoracic, and in ordinary Podophthalmia by all 
the cephalic and thoracic somites. 
9. Lastly, there are certain parts developed singly in the median line in the Articulata. 
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 labium or lingua) of Crustacea, and the lingua of 
Insecta, 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 Articulata. 
§ 5. The Embryogeny of the Articulata, Mollusca, and Vertebrata compared. 
I find it difficult to conclude this memoir without saying a few words on the resem- 
blances and differences between the embryogenic changes of the Articulata and those of 
the Mollusca and Vertebrata. Absolute and fundamental differences appear to me 2 
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, 
80 far as I am aware, it is the opposite or hæmal side which is first formed in every 
* “Lectures,” Med. Times and Gazette, 1857. 
