360 
BULLETIN OE THE UNITED STATES FISH COMMISSION. 
bahamensis Oliv., and often on the carapace of other crabs, such as Pericera cornuta Latreille. The 
latter relationship seems of very constant occurrence. The spinous crab is rarely found without 
many specimens of different sizes of the sea anemone adhering to its carapace and limbs. Pyrula 
shells inhabited by a Petrochirus are at times almost completely hidden by the anemones. As many 
as fourteen polyps have been found on a single shell, occupying almost every position, some even within 
the mouth, where they must come into close association with the hermit crab. 
The polyps usually remain in an expanded condition when alive and appear very hardy, living 
well in aquaria. They retract quickly upon irritation, the disk, tentacles, and upper part of the column 
being infolded, but open again almost as readily. The lower part of the body is capable of considerable 
distention. The column may elongate and then be deeply constricted across the middle. 
After two or three days’ confinement in an aquarium the thick cuticle loosens and peels off, and 
the scapus then appears smoother. Should the water become impure the polyps are able to detach 
themselves and forsake the Surface of the shells, move away and remain free, or they may again fix 
themselves by their expanded base to the smooth surface of the vessel. On irritation of the polyps 
acontia are emitted freely through the mouth, and apparently less freely through the cinclides. The 
threads are capable of independent movement after being shot out and can be again indrawn. 
External characters . — The base of the polyps is usually spreading and much broader than the 
column. Generally a horny cuticular secretion intervenes between the ectoderm and the surface of 
attachment. The base is circular in small specimens, but in larger examples becomes very irregular 
in outline, adapting itself to the spiral convexities of the shell to which it adheres. The wall is so 
thin that the lines of attachment of the mesenteries show through, and is highly distensible when the 
polyp happens to leave its attachment. 
The column is short, erect, broad below 7 , narrowing above the middle and enlarging again toward 
the apex. The proximal part is very irregular in outline, while the middle and upper are circular in 
section. The wall appears thick and coriaceous, especially after preservation. The division into scapus 
and capitulum is readily observed in expanded polyps, the surface of the scapus being usually coarse 
and wrinkled, while that of the capitulum is smooth and more delicate. The coarseness of the former 
is largely due to the presence of an outer, loose, cuticular investment to which foreign particles adhere, 
and which is continually being shed in confinement. Often the column presents longitudinal and 
transverse furrows which give rise to approximately square or rhomboidal areas, more or less regularly 
arranged, or sometimes it presents distinct tubercles irregularly scattered. The lines or furrows are 
usually much less marked in living than in contracted preserved specimens. 
Two or three alternating cycles of slightly elevated cinclides occur a short distance from the base. 
In the lower cycle they are smaller, and generally twice as many as in the next cycle. Only two or 
three members occur in the third uppermost cycle, but these are the larged of the series. They are 
easily distinguished in the living polyp by their darker color, and project above the general level of 
the surface. The apertures vary much in number and regularity of arrangement; sometimes certain 
members may be missing from the cycles, or additions may be intercalated at almost any part of the 
cinclidal zone. On irritation white or salmon-colored acontia maybe extruded through the cinclides, 
and also through the mouth, as well as through accidental perforations in the disk. 
The capitulum appears as a narrow, circular, smooth band at the apex of the column, and is 
more diaphanous than the scapus. 
The tentacles are short, smooth, subulate, eutacmaeous, closely arranged, and overhanging. In 
mature polyps they number 192, or even more, arranged according to the formula 6 6 12 18 96. 
Irregularities or variations from the hexameral plan sometimes occur. 
In expanded polyps the periphery of the peristome is slightly depressed, thin-walled, circular, 
and grooved in correspondence with the radiating, internal attachment of the mesenteries. The 
central area surrounding the mouth is usually elevated. The mouth is very small, and circular or 
oval in outline; the lips project but little, and the two gonidial grooves are not readily seen. When 
much expanded the mouth becomes larger and slit-like, or the stomodseum may be partly protruded. 
The gonidial groove is then clearly indicated at each end, and the stomodaeal walls are seen thrown 
into six very strong folds on each side. When alive the lateral walls of the stomodaeum sometimes 
meet along the middle, leaving open the grooves at each end, through which currents of water may 
be established. 
