244 robert h. wolcott: 



BIOLOGICAL CONSIDERATIONS. 



Most of these mites pass all their life stages within the 

 limits of the mussel's shell, moving about over the surface of 

 the mantle; a few, of which A. crassipes is an example, remain 

 only till the last stages of development are reached and then 

 leave the shelter of the mussel to pass the remainder of their 

 life swimming freely about in the water. The eggs are depos- 

 ited singly in the mantle and gills, rarely, as in A. (JST.)'mg.ens i 

 in masses between the gills. The spines guarding the genital 

 opening, by being inserted into the surface of the gill, prob- 

 ably assist in oviposition, either by holding the body firmly 

 against the surface of the gill or mantle which is then pierced 

 by the ovipositor, or by themselves piercing the surface, the 

 ovipositor being protruded between them and the eggs de- 

 posited, rather more abundantly in the gills apparently, in the 

 case of the CWo-dwellers, more numerously in the mantle by 

 the ^L??<9<&w&2-parasites. The development of these eggs has 

 been studied by Ciaparede (68) and others and five stages hive 

 been distinguished. 



First, the egg stage, in which the embryo is surrounded by 

 a firm outer membrane and a more delicate inner membrane 

 — the " Zwischenhaut " of Ciaparede (68), the " Dotterhaut " 

 of Kramer (80) or the "Apoderm" of Henking (82). The 

 former, as development proceeds and the embryo increases in 

 size, is ruptured, but the latter increases in size or is capable 

 of distension and remains surrounding the embryo till the young 

 mite is ready to emerge, when it is also burst open and the larva 

 appears. Third, the first larval stage, in which the mite has but 

 three pairs of legs, and which is very short. Fourth, the nymph 

 stage, in which the larval legs are lost and a new set is de- 

 veloped beneath the old larval skin. Fifth, the second larval 

 or sub-imago stage, in which the mite has four pairs of legs but 

 in which it lacks still the perfect development of the sexual ap- 

 paratus and the relative proportions belonging to the adult, 

 those only being acquired when a final moult of the skin re- 

 veals the adult mite. It is not the intention in this paper to 

 go into details as to this process, further than to note that the 



