AND MORPHOLOGY OF APHIS. 227 
series of similar appendages, all elongated and bent back against the sternal surface of 
the body. 
The abdomen is very short, but is clearly distinguishable from the thorax by its less 
complete segmentation, and by the rudimentary condition of all its appendages save the 
last pair. 
The blastoderm as yet extends only for a little way on the sides of the body. The pri- 
mitive larval integument still invests the whole body loosely, but passes smoothly over’ 
all the appendages, except the antennules and antennæ, which continue to be ensheathed 
by it. 
The larva remains in this general condition until it attains th of an inch in length, 
the principal differences in its later stages being the increased growth of the body as com- 
pared with the head, the completion of the dorsal surface by the upward extension of the 
blastoderm, and the gradual restriction of the yelk to the anterior part of the body. 
I have been unable to determine, as precisely as in Aphis, the exact share taken by the 
procephalic lobes in the composition of the roof of the head in the crustacean; but they 
assuredly extend over a considerable part of its latero-dorsal parietes. 
The carapace appears at first as a ridge-like process developed from the dorso-lateral 
region of the antepenultimate and preceding thoracic and cephalic somites, as far for- 
wards as the bases of the antennules. It is certainly not an extension backwards of the 
terga of any of the anterior cephalic somites, but is from the first continuous with, and 
developed from, the thoracic somites. 
It is needless to trace the history of the larval Mysis further, —what has been said suf- 
ficiently proving the close resemblance of its development to that of Aphis. 
$ 3. Embryogeny of Scorpio as exemplifying Arachnida. 
I have not yet had the opportunity of working out the development of an Arachnidan ; 
but the researches of Rathke* and Heroldt are so full and clear, that the omission is of 
little moment. 
Rathke's observations on the development of the Scorpion show that after, or even 
before, the blastoderm has extended over the whole yelk, a papillary elevation appears at 
one pole. It is the rudiment of the future abdomen, including under that term all the 
segments of the body behind that which carries the last pair of respiratory organs. In 
front of this, eleven pairs of closely approximated thickenings make their appearance; and 
then, at the sides of the sixth to the tenth pair of them, inclusively, counting from the 
rudimentary abdomen, papillary processes are developed. It is clear, from Rathke’s 
figures, that the anterior pair of thickenings are the “ procephalic lobes,” while the succeed- 
ing ones are the sterna of the somites between the mouth and the abdomen. The five pairs 
of processes thrown out by the five anterior of these are the great chelæ and the four pairs 
of ambulatory appendages. The antennæ make their appearance subsequently from the 
procephalic lobes (or their junction with the rest of the blastoderm) in front of the mouth. 
Tt is not expressly stated, but I do not doubt, from Rathke’s figures, that the upper region 
Of the head is formed, as in Insecta and Crustacea, by the union of these lobes. 
* Reisebemerkungen aus Taurien, 1837. + De Generatione summ d 
VOL. XXII. 
