444 ACARINA. 



The four pairs of provisional appendages exhibit no signs of 

 atrophy ; and the extent of the ventral flexure is shewn by the 

 angle formed between the line of their insertion and that of the 

 appendages in front. The yolk has enormously distended the 

 integument between the two halves of the ventral plate, as is 

 illustrated by the fact that, at a somewhat earlier stage than 

 that figured, the limbs cross each other in the median ventral 

 line, while at this stage they do not nearly meet. The limbs 

 have acquired their full complement of joints, and the pedipalpi 

 bear a cutting blade on their basal joint. 



The dorsal surface between the prominent caudal lobe and 

 the procephalic lobes forms more than a semicircle. The terga 

 are fully established, and the boundaries between them, especially 

 in the abdomen, are indicated by transverse markings. A large 

 lower lip now bounds the stomodseum, and the upper lip has 

 somewhat atrophied. In the later stage (fig. 201 B) the greater 

 part of the yolk has passed into the abdomen, which is now to 

 some extent constricted off from the cephalo-thorax. The 

 appendages of the four anterior abdominal somites have dis- 

 appeared, and the caudal lobe has become very small. In front 

 of it are placed two pairs of spinning mammillae. A delicate 

 cuticle has become established, which is very soon moulted. 



Acarina. The development of the Acarina, which has been mainly 

 investigated by Claparede (No. 446). is chiefly remarkable from the frequent 

 occurrence of several larval forms following each other after successive 

 ecdyses. The segmentation (vide p. 116) ends in the formation of a blasto- 

 derm of a single layer of cells enclosing a central yolk mass. 



A ventral plate is soon formed as a thickening of the blastoderm, in which 

 an indistinct segmentation becomes early observable. In Myobia, which is 

 parasitic on the common mouse, the ventral plate becomes divided by five 

 constrictions into six segments (fig. 202 A), from the five anterior of which 

 paired appendages very soon grow out (fig. 202 B) The appendages are the 

 chelicerae (cJi) and pedipalpi (pd} and the first three pairs of limbs (p l fl 3 ). 

 On the dorsal side of the chelicerae a thickened prominence of the ventral 

 plate appears to correspond to the procephalic lobes of other Arachnida. 

 The part of the body behind the five primitive appendage-bearing segments 

 appears to become divided into at least two segments. In other mites the 

 same appendages are formed as in Myobia, but the preceding segmentation 

 of the ventral plate is not always very obvious. 



In Myobia two moultings take place while the embryo is still within the 

 primitive egg-shell. The first of these is accompanied by the apparently 

 total disappearance of the tliree pedifonn appendages, and the complete 



