368 



ACARINA. 



Acarilia. The development of tlie 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. 95) 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 chelicerse (ch] and pedipalpi (pel) and the first three 

 pairs of limbs (p l p 3 ). On the dorsal side of the chelicerre 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 three pediform appendages, and the complete 

 coalescence of the two gnathiform appendages into a proboscis (fig. 202 C). 

 The feet next grow out again, and a second ecdysis then takes place. 

 The embryo becomes thus enclosed within three successive membranes, 



FIG. 202. FOUR SUCCESSIVE STAGER IN THE DEVELOPMENT OF MYOBIA 



MUSCULI. (After Claparede.) 



s 1 s 4 . post-oral segments; ch. chelicerae; pd. pedipalpi; pr. proboscis formed by 

 the coalescence of the cheliceraj and pedipalpi ; p 1 , p*, etc. ambulatory appendages. 



viz. the original egg-shell and two cuticular membranes (fig. 202 D). 

 After the second ecdysis the appendages assume their final form, and the 

 embryo leaves the egg as an hexapodous larva. The fourth pair of ap- 

 pendages is acquired by a post-embryonic metamorphosis. From the pro- 



