ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 75 
terga which belong to the two posterior pairs of ambulatory limbs 
are clearly visible on the posterior part of the cephalothorax, and 
these limbs are strongly distinguished from the anterior two pairs by 
the absence of the triangular processes directed towards the mouth, 
which characterize the bases of the latter. Indeed, the anterior am- 
bulatory legs, by means of these processes, take part in the formation 
of the oral cavity as completely as do the maxilla of any other 
Articulate animal. 
Another exceedingly natural demarcation between the two anterior 
and two posterior pairs of ambulatory limbs is afforded by the so- 
called ‘diaphragm’ which divides the thoracic from the cephalic 
cavity, and whose attachment corresponds with the interval between 
these two sets of appendages. 
In Galeodes, the two posterior pairs of ambulatory legs are attached 
to distinct segments. 
There is no necessity to enter into any disquisition upon the 
homology of the appendages and the general uniformity in plan, so 
far as the anterior part of the body is concerned, 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 Lemodtpoda 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 Artculata 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 modification 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 Sguzl/a is assuredly developed from not 
more than four somites, the antennary, mandibular, and mawillary. 
In Agus, I doubt whether more than the six cephalic somites enter 
into its composition. In Cuma it is constituted by the cephalic and 
1 «Lectures,’ Med. Times and Gazette, 1857. 
