38 THE AMERICAN NATURALIST. [Vor. XXXIX. 
encrusted on a female Pinnixa, may be regarded as an accidental 
enclosure. 
The tubes of Chatopterus are not the only abodes of some of 
the commensals named : Nereis is found among the shells, rocks 
and sea-weeds ; Pinnotheres maculatus, in the shells of Pinna 
semi-nuda like the related species, P. ostreum, in the oyster ; 
whilst Menippe commonly keeps in hiding in crevices and pits in 
rocks and shells. So far as I know, Polyonyx macrocheles and 
Pinnixa chetopterana, beyond the young stages, are only very 
rarely found free on the shoals, but Polyonyx ocellata is found at 
Beaufort on the body of Limulus, and sometimes in old shells 
dredged outside the harbor. 
The sizes of the crabs range from two millimeters in width 
to that of the full-grown animal (thirteen to fourteen millimeters 
wide), and the smallest ones are of such a size that they could 
-readily pass through the orifices of the tubes, and they probably 
do so for I have frequently collected tubes with two adult crabs 
and a single small one but rarely with three full-grown indi- 
viduals. 
. The position of the crabs in the interior of the tubes I learned 
by keeping the Cheetopterus and crabs in glass U-tubes, to the 
open ends of which the annelids constructed inverted parchment 
funnels, which prevented their escape. While the commensals 
move about with rapidity they remain at one end of the U-tube, 
usually the one opposite that occupied by the annelid itself, but 
when the annelid reverses its position they press past it to the 
opposite end and there remain bathed in the passing current. 
The annelids, Nereis, moved along the dorsal wall of the tube 
and did not interfere with Chstopterus which occupied the 
ventral portion of the tube. 
The commensals, which are usually found near the orifices of 
the U-tube, are advantageously located for securing food, which, 
if it consists either of vegetable matter or of copepods and 
various larvae which pass through the tube in the moderate cur- 
rent of water, and the worms that may seek shelter in the tube, 
is very abundant. 
The commensals are permanently confined within the tubes 
of Chzetopterus where the breeding is comparatively simplified 
