TRACK EAT A. 



445 



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 inclosed within three successive membranes, 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 appendages is 



FIG. 202. FOUR SUCCESSIVE STAGES IN THE DEVELOPMENT OF MYOBIA MUSCULI.- 



(After Claparede.) 



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

 the coalescence of the chelicerse and pedipalpi ; p l , /*, etc. ambulatory appendages. 



acquired by a post-embryonic metamorphosis. From the proboscis are 

 formed the rudimentary palpi of the second pair of appendages, and two 

 elongated needles representing the chelicerae. 



In the cheese mite (Tyroglyphus) the embryo has two ecdyses which are 

 not accompanied by the peculiar changes observable in Myobia : the 

 cheliceras and pedipalpi fuse however to form the proboscis. The first 

 larval form is hexapodous, and the last pair of appendages is formed at a 

 subsequent ecdysis. 



In Atax Bonzi, a form parasitic on Unio, the development and meta- 

 morphosis are even more complicated than in Myobia. The first ecdysis 

 occurs before the formation of the limbs, and shortly after the ventral plate 

 has become divided into segments. Within the cuticular membrane resulting 

 from the first ecdysis the anterior five pairs of limbs spring out in the usual 

 fashion. They undergo considerable differentiation ; the chelicerae and 

 pedipalpi approaching each other at the anterior extremity of the body, and 

 the three ambulatory legs becoming segmented and clawed. An oesophagus, 

 a stomach, and an oesophageal nerve-ring are also formed. When the larva 



