640 ARACHXIDA. 



feet (though in others at this stage there are four pairs), and 

 after another moulting the fourth pair of limbs appear. The 

 young mite is analogous to the "Nauplius" stage of many 

 low Crustacea. 



Claparede* has observed in Atax Bonzi, which is a parasite 

 on the gills of fresh-water mussels, that out of the originally 

 laid egg (Plate 11 fig. 3, embryo of Atax Bonzi ; 7c, head-plate ; 

 ag^ infolding of the belh' ; fZm, intermediate skin; mo, outer 

 shell of the egg; mxl., mandibles; jnx, maxillre ; p^-p'^-, legs; 

 vt, yolk. Fig. 4, front view of the same) ; not a larva, but 

 an egg-shaped form hatches, which he calls a "dcutovum." 

 (PI. 11 fig. 1, bursting of tlie egg-shell into two halves, mo, on 

 the day that the dentovum, dm, hatches out; md, mandibles: 

 mx, raaxilUe ; jo^, third pair of legs ; Ui, body cavitv ; -^J)-) com- 

 mon beginning of the alimentary canal and nervous system ; 

 ami, hsemaboeba, amoeba-like bodies, which represent the blood 

 corpuscles ; there being no cn'culation of the blood, the move- 

 ments of the ha^maboeba constitute a vicarious circulation. 

 Fig. 2, the dentovum free from the first egg-shell ; lettering 

 same as in Fig. 1, oc, rudiments of the simple eyes; r, beak; 

 h, h', rudimentary stomach and liver). From this dentovum 

 (which is not the "amnion" of insects) is developed a six- 

 footed lar\'a. This larva passes into an eight-footed form, the 

 "second larva," (the "nymph" or pupa, of Dujardin and 

 Robin) whicli transforms into the adult mite. The pupa dif- 

 fers from the adult in having longer feet, and four instead of 

 ten genital cups, the latter being the usual number in the adult. 



The larvjTi are elongated oval, with six long legs and four 

 ocelli. They swarm for a short time over the gills of the mus- 

 sel tliey are living on and tlien bore into the substance of 

 the gill to undergo their next transformation. Here the young 

 mite increases in size and l)ecomes round. The tissues soften, 

 those of the diflTerent organs not being so well marked as in 

 the first larval stage. The limbs are short and much largei 



*The flevelopnient of spirlers anrt of tlie Ar.achnifls prcnerallv, has been traccfl 

 liy Rathke, Ilerolil, and more espe;-,ially by Claparede, in a work of great ability, 

 from whicli we liave ilrawn the jtre-edintr account, often nsinjr the anthor's own 

 words. Hi^ observations were nnde on virions genera of spiders (Pholcus, etc.) 

 His "Studies on Mites," from Avhicli Plate II is copied, appeared in Siebold's and 

 KOlliker's Journal of Scientific ZoOlogy, 18::8, part iv. 



