1950] 
Humes and J amnback — Parasitic Water-mite 
83 
two outer gills as marsupia. In the 17 positive Lampsilis radiata 
the mites occurred nearly equally in the four suprabranchial cham- 
bers. One clam had mites in all four of its suprabranchial chambers. 
Ninety-one per cent of the parasitized clams had papillae of var- 
ious sizes on the walls of the suprabranchial chambers. As many 
as 40 papillae were counted in one suprabranchial chamber contain- 
ing a pair of mites. The largest papillae measured were 3 mm. in 
diameter and 5 mm. in length. Fig. 4 shows a specimen of Anodonta 
cataracta from which the left valve and mantle have been removed. 
The left outer suprabranchial chamber has been opened to show the 
papillae, a male and a female N. ingens, and several small, dark egg 
masses. In only 10 clams were papillae found without mites. Pre- 
sumably these clams had previously been parasitized by N. ingens. 
The papillae were often arranged in an incomplete ring around the 
mites and their egg masses, perhaps acting as a barrier to prevent 
the eggs from being carried out of the gill chambers, either via the 
excurrent siphon in Anodonta or into the mantle cavity by way of 
the incomplete connection of the innermost gill lamella and visceral 
mass in Elliptio. The papillae occur mostly within the supra- 
branchial chambers, but in a few cases in E. complanatus extended 
down on the side of the visceral mass. 
The presence of the mites apparently interferes with the use of 
the gills as marsupia. In many gravid, parasitized A. cataracta the 
marsupia containing mites were asymmetrically developed. The 
anterior half of the parasitized gill contained few or no glochidia, 
while the unparasitized gill of the opposite side was normally de- 
veloped. 
The factors involved in regulating the geographical distribution 
of N. ingens are difficult to determine. The mites occurred in small 
ponds, large lakes, and in slowly flowing streams. All clams ex- 
amined were taken from a depth of not more than 4 feet. The types 
of bottom included soft clay, silt, fine white sand, gravel, or small 
stones. 
Both N. ingens and species of Unionicola were occasionally found 
in the same clam host. Adults of the two genera are easily differ- 
entiated, Najadicola being larger, cream-colored, and remaining 
within the suprabranchial chambers, while Unionicola is smaller, 
dark-colored, and crawling about over the surface of the gills and 
mantle. 
The eggs of N. ingens are laid in masses in the suprabranchial 
