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 stiU invests the whole body loosely, but passes smoothly over 

 aU the appendages, except the antennules and antennae, which continue to be ensheathed 

 by it. 



The larva remains in this general condition until it attains i^jth 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 imable 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-Mke 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 Ai'achnidan ; 

 but the researches of Rathke* and Heroldf are so fuU 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, iucluding imder that term aU the 

 segments of the body behind that which carries the last pair of respii-atory organs. In 

 front of this, eleven pairs of closely approximated thickeniags 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, fi-om Eathke's 

 figm-es, 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 chelaj and the four pairs 

 of ambulatory appendages. The antennae make their appearance subsequently from tbe 

 procephalic lobes (or their junction with the rest of the blastoderm) in front of the mouth. 

 It 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 TaurieD, 1837. t De Generatione Aranearum, 1824. 



VOL. XXII. 2 H 



