Contributions from the Biological Laboratory of the U. S. Fish Commission, 
Woods Hole, Massachusetts. 
A NEW ISOPOD PARASITIC ON THE HERMIT CRAB. 
By MILLETT T. THOMPSON. 
While at Woods Hole, in the summer of 1897, studying the small hermit crab 
(Pagurus loru/icarpus Say), 1 found parasitic upon this crustacean a hitherto unde- 
scribed Bopyrid, allied to Phryxus resupinatus Muller, and apparently representing 
a new genus. About 1.5 per cent of the crabs at Great Harbor were thus infested 
(1898), and from 8 to 1 per cent of those at Hadley Harbor. A single specimen was 
taken at Edgartown in 1898, and another at Warwick, Ehode Island, in 1900. In the 
channels where there is a swift current the percentage of infested crabs is low, due 
probably to the more effectual dispersal of the free-swimming larva?. 
The female parasite occurs on the abdomen of the hermit, to which it is attached, 
back downward, by its mandibles and legs. The male is found on the posterior part of 
the marsupium of the female, usually lying to the right (apparent left) of the median 
line, the head directed anteriorly. The presence of the parasite does not effect any 
alteration in the case of the secondary sexual characters of its host. On the average 
the infested crabs seem quite as resistant to adverse conditions as the uninfested ones. 
Stegophryxus hyptius, genus et species nov. 
Adult Female (Plates 9 and 10). 
Broad in proportion to length, marsupium very large, abdomen about half the length of thorax, 
distinct from it, 6-joinl;ed, with five pair of triramous pleopoda and a pair of oval uropoda. Length, 
about 9.1 ram. Color, yellowish-white, opaque. Ovaries, when full of ripe eggs, orange-yellow. 
Head (pi. 9, figs. 5 and 6), from the dorsal side appears as an oblong elevation ending anteriorly 
ilia blunt lobe, which represents the median portion of the much-reduced front (Fr). As the lateral 
portions of the front are almost wholly obsolete, appearing only as two inconspicuous lobes, the 
greater part of the antennules, antennae, and tip of rostrum, is visible dorsally. The antennules are 
3-jointed and consist of a large globose basal joint, surmounted by a small cylindrical second and a 
minute third joint; outer joints bristle-tipped. Each antenna (A 2 ) arises along the side of rostrum as 
a columnar ridge, whose distal end is visible dorsally (fig. 7, A 2 ). From this ridge a 4-jointed 
flagellum arises, its proximal joint stout, the three distal joints slender; all the joints bristle-tipped. 
Ventral surface of head broader than dorsal surface and sharply elevated at posterior border, giving a 
strong antero-dorsal slope so that the erect hypopharynx points almost anteriorly. At sides of pos- 
terior border three curved processes arise (fig. 7, Pro), and in the midline are two thin foliaceous plates 
(fig. 7, Fp). Rostrum conical. Mandibles (Mncl) slender, with expanded tips, the edges of which are 
incurved so that, pressed together, they form a sucking-tube. Near the bases of mandibles appear the 
oval maxilluke {Mx x ). Hypopharynx (fig. 7) erect, highly keeled, and plays no part in formation of 
rostrum. Maxillipeds (fig. 5) large; each consists of a foliaceous anterior and a somewhat thicker 
posterior blade; during life these organs keep up a rapid fanning motion. There is no trace of a palpus. 
The thorax (pi. 9) is concealed ventrally by an enormous marsupium, built up of five pairs of 
53 
