330 



THE ANATOMY OF INVERTEBRATED ANIMALS. 



vided with only three pairs of ambulatory limbs, the fourth 

 pair making its appearance only after ecdysis has occurred. 



Fig. 90. — Ixodes vicmus, female (after Pagenstecher 1 ).— a, mandibular hooklets ; b, 

 d, e, fourth, third, and second joints of the palp ; c, hooklets of sternal surface of 

 proboscis ; /; base of the proboscis ; g, stigma ; A, genital aperture ; i, anal 



valves. 



In some Acarina, a singular kind of metamorphosis 

 occurs. 



Thus, in Atax Bonzi, Claparede 2 observed that, before 

 the limbs appear on the blastoderm, a chitinous cuticula is 

 separated and forms an envelope, which he terms the "sac 

 of the deutovum." The proper vitelline membrane bursts 

 into two halves, much as in Limulus, and the deutovum 

 emerges. In the mean while, the anterior end of the blasto- 

 derm becomes fashioned into two procephalic lobes ; while 

 five pairs of tubercles, answering to the rudiments of the 

 chelicerre, pedipalpi, the two posterior gnathites, and one 



1 " Anatomie der Milben," 1860. 



2 " Studien an Acariden." (Zeitsdirift fiir wus. Zoologie, 1868.) 



