98 



ARACHXIDA. 



-4TA 



where it appears more like a slit (at the left side of the inner 

 envelope). Claparede thought that the tooth served for splitting 



the envelopes. It thus 

 /iee£. cA . performs the same func- 



tion as the egg-tooth of 

 the Araneae (p. 58), but 

 it need hardly be pointed 

 out that the difference 

 in position of the two 

 structures makes it im- 

 possible to homologise 

 them. We might rather 

 compare the structure 

 just described with the 

 egg-tooth of theOpiliones 

 (p. 33). 



The embryo, sur- 

 rounded by the cuticular 

 membrane, emerges from 

 the egg-shell (Fig 50 D), 

 which, however, con- 

 tinues to surround the 

 greater part of it. This recalls the cuticular membrane in the 

 Araneae, which forms under the egg-shell and encloses the hatched 

 and still motionless embryo. The limbs now grow out again, but 

 are reduced as before, and a second cuticular integument is cast off, 

 so that the greater part of the egg is enclosed by two integuments as 

 well as by the egg-shell. The tritovum of Claparede is thus formed. 

 Within it the embryo attains the six-limbed form in which it finally 



Fig. 52. — The larva of Trovibidium with six limbs, en- 

 closed in the deutovum (after Henking). abd, abdomen ; 

 ch, chelicerae ; d, yolk (enteron) ; dm, deutovum ; 

 Pi - ?'.-!. fi rs t to third pair of limbs ; ped, pedipalps ; st, 

 "stigma"; ut, "primitive trachea"; z, isolated cells 

 beneath the deutovum. 



emerges. 



In the secretion of two envelopes within the egg in Myobia we 

 have a specially complicated process. So far as is known, only one 

 such envelope usually forms in the egg (Fig. 51). We must probably 

 regard the formation of these envelopes as a very early moult, which 

 no doubt originally took place during larval life. This view is 

 supported by the fact that, in the further course of development, 

 several similar moults occur. The embryo maj r also actually leave 

 the egg surrounded by this first larval integument. In Myobia, 

 Damaeus, etc., the egg-shell is only split so as to allow a part of the 

 "deutovum" to emerge (Fig. 50 D), but in many other Acarina, 

 e.g., Atax and Trombidium, the egg-shell is quite cast off, and the 



