640 ARACHNIDA. 



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

 after another moulting the fourth pair of limbs appear. The 

 3'oang 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 ; ?c, head-plate ; 

 ag, infolding of the belly ; dm, intermediate skin ; mo, outer 

 shell of the egg ; md, mandibles ; mx, maxillae ; 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 "deutovum." 

 (PI. 11 fig. 1, bursting of the egg-shell into two halves, 7no, on 

 the day that the deutovum, dm., hatches out ; md, mandibles : 

 mx, maxillae ; p\ third pair of legs ; Ih, body cavit}^ ; sp, com- 

 mon beginning of the alimentar^^ canal and nervous system ; 

 cmib, haemaboeba, amoeba-like bodies, which represent the blood 

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

 ments of the haemaboeba constitute a vicarious circulation. 

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

 same as in Fig. 1, oc, rudiments of the simple ej-es ; r, beak; 

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

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

 footed larva. This larva passes into an eight-footed form, the 

 "second larva," (the "nj-mph" or pupa, of Dujardin and 

 Robin) which 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 lar^'se are elongated oval, with six long legs and four 

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

 sel they are living on and then bore into the substance of 

 the gill to undergo their next transformation. Here the young 

 mite increases in size and becomes round. The tissues soften, 

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

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



* The development of spiders and of the Arachnids generally, has been traced 

 by Rathke, Herold, and more especially by Claparede, in a work of great ability, 

 from which we have drawn the lu-eccding account, often using the author's own 

 words. His observations were made on various genei-a of spiders (Pholcus, etc.) 

 His "Studies on Mites," from M'hich Plate 11 is copied, appeared in Siebold's and 

 Kolliker's Journal of Scientific ZoOlogy, 1868, part iv. 



