A NEW ISOPOD PARASITIC ON THE HERMIT CRAB. 
rudimentary. Abdomen as long as thorax. Length, 5 mm. Head short. Front ( Fr ) prominent, 
transverse, with a straight, entire, uninflexed margin, which conceals the month parts, antennules, 
and the basal joints of antennae. Mouth parts much like those of adult, but the hypopharynx is flatter 
and less erect and the mandibles are stout and have sharp tips ( Mud ). Indexed border of labrurn 
narrow, maxillipeds small and not inflated at base. They consist of an oblong-oval posterior and a 
smaller rounded anterior blade. No palpus. The rostrum points anteriorly, rather than, as in the 
adult, dorso-anteriorly. Thorax narrow and not quite as long as abdomen. Segments subequal, 
fleshy, with the median and lateral parts of nearly same width (fig. 9). Sixth and seventh segments 
narrower than the rest, distinct from them, and have only rudiments of ventral keels. Epimera (?) 
of thorax distinct ( Ep ), no “cushions” or “shields.” Pereiopoda like those of adult. Brood-plates 
small and flat; those of third and fourth pairs are without dorsal portion, those of fifth pair lack 
pouches at posterior angles. The abdomen is like that of adult in form, except that the ventral rami of 
first pair of pleopoda are not expanded ( V pl^). 
II. Three cryptoniscid-stage larvae, probably males, were taken from the female just described. 
One was in the marsupium; the others were clinging to the appendages (pi. 10, figs. 3, 4, and 5). 
Abdomen proportionally longer than in adult and consisting of 0 segments. Pleopoda and uropoda 
present. Epimera distinct. Color white with black blotches. Length about 0.7 mm. Margin of 
head narrow, indexed at sides and in front. Rostrum prominent, Antennules complex in structure 
and provided with long bristles (fig. 4, A.i\). Antennae 8-jointed and very long and slender. Thorax 
of 7 smooth segments, with distinct strongly deflexed epimera (Ep); 7 pairs of pereiopoda similar in 
form to those of adult, but more delicate and slender. The abdomen consists of 6 segments, is highly 
convex, the first 5 segments having deflexed epimera. Five pairs of flat biramous pleopoda (fig. 5), 
the blades standing with their faces at right angles to the long axis of body. Uropoda (fig. 3, Ur) 
biramous, consisting of a cylindrical protopodite, a cylindrical exopodite, and a shorter cylindrical 
endopodite. Each endopodite and exopodite bears one long bristle and a tuft of short hairs. 
III (not figured). Several females, of a stage considerably more advanced than the one described 
above, were taken. In all cases they had a male of about 2 mm. in length, of adult form, clinging 
to them. Length varying from 6 mm. to 7 mm. Appearance much like fig. 9 of plate 10, but the 
whole thorax is wider. Lateral parts slightly broader than median portion. The head is adult in 
length, but the front is still rather wide, very fleshy, and trilobed, the lateral lobes being larger than 
the median. Tip of rostrum and distal portions of an ten nuke and antennae visible dorsally; otherwise 
the head and mouth parts are adult in structure. Thorax narrow and as long as abdomen. Dorsal 
segments fleshy, though less so than in preceding stage, and the first five segments have begun to 
crowd anteriorly. Sixth segment fleshy, not keeled, scarcely longer than fifth or seventh. Ventrally 
the transverse keels on sixth and seventh segments are rudimentary, but more developed than in 
metaphryxoid stage; marsupium larger, though the brood-plates are still quite rudimentary. Brood- 
plates of third and fourth pair have developed the dorsal portion; first pair nearly adult in shape, and 
the funnel under the head has begun to form. Tips of first brood -plates and maxillipeds are visible 
from dorsal side, much as in adult. The pereiopoda are adult in form, and “cushions” and “shields” 
are present. Abdomen like that of the metaphryxoid stage. 
IV (not figured). One specimen of a female nearly mature was taken. In this the abdomen was 
nearly as long as the thorax, sixth thoracic segment not yet of adult proportions. Length, 10 mm. 
As mentioned above, the nearest relative of this species is Bopyrus ( Phryxus ) resupinatus Muller, 
described in 1870 from a small hermit-crab at Desterro, South America. This parasite was attached, 
in its early larval stage at least, at the roots of Sncculina purpurea, or less frequently Peltogaster sociulis, 
both of which Cirri peds were extremely common on the hermit-crab mentioned, the name of which is 
not given. This is strikingly unlike the condition in regard to' Stegophryxus hyptius, for I have never 
taken the latter in association with any other parasite. I have examined several thousand specimens 
of Pagurus longicarpus from the vicinity of Woods Hole, but have never found any external parasite 
other than the Bopyrid. The proportions and structure of body of the Desterro species are also different 
from those of Hyptius , as far as can be judged from the very imperfect knowledge of the anatomy of 
resupinatus at our disposal. Hence I consider the two forms to be distinct, though the likeness of the 
Woods Hole form to Muller’s species suggested the name hyptius (vTCvio<;=resupinalus) . 
As will be noted, the male is similar to the male of Athelges (Hesse), Pleurocrypta (Hesse), and 
Phryxus (Riithke) , and on this basis apparently some authors have grouped the species of these three 
genera under Phryxus. Phryxus thus defined will of course admit hyptius and resupinatus, but when we 
