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The Trematopn Larva and Ascaris of the Carcinus M@NAS. 
By W. C. M‘Inrosu, M.D., F.L.S. 
(With Plate VIII.) 
Carcinus menas is pre-eminent amongst the higher Crus- 
tacea for its marauding habits; but it pays a heavy penalty 
in becoming the abode of numerous parasites. The young 
of the Common Mussel attach themselves to the eye-sockets, 
and, appropriating the cavities, cause the destruction of the 
eyes, which, protruding, become a site for Membranipora 
pilosa, or fall off. The same shell-fish attaches itself (some- 
times in numbers) to the central groove on the inferior sur- 
face of the cephalo-thorax, binding the tail thereto by its 
byssus, and proving more troublesome in that situation than 
a large Pachybdella. It occasionally blocks up the branchial 
slits, or seriously interferes with the functions of the respira- 
tory organs, by lodging in the branchial cavity, and fixing the 
external whip (of the third pair of foot-jaws) to the outer 
wall by its threads. The pits of the internal antenne also 
suffer from their inroads, so as sometimes to cause the debat- 
able functions of this organ to be sadly impeded. In a large 
male Carcinus, for example, there were ten small Mussels 
under the abdomen, one blocked up the left anterior branchial 
slit, and the right eye was unhoused and projecting by another 
of these curious molluscs. The crab also bore Balani on 
various parts, a considerable patch of Lepralia hyalina on the 
left under side, a compound Ascidian covered most of the 
corresponding right region, and extended, in company with 
Membranipora pilosa, over the dorsum. ‘These are but a few 
of the external parasites of this active Crustacean. 
With so many external parasites, the occurrence of numer- 
ous, if not different, internal ones, is perhaps less surprising. 
In dissecting out the nerves of a specimen minute specks 
were found on the branches of the great thoracic ganglion, 
adhering apparently to the sheaths of the nerves and following 
them in their distribution, occasionally in groups of two and 
three. The same bodies also abounded in the liver. Undera 
lens they were seen to be small glassy ova, with opaque white 
internal markings. In upwards of forty Carcini examined 
they have been found in every specimen, irrespective of sex, 
from three fourths of an inch across the carapace upwards. 
In well-marked cases they occur in hundreds, attached to the 
ducts and blood-vessels of the liver, between the papillz, and 
crowded together like clusters of grapes. Scarcely a soft 
texture in the interior, with perhaps the exception of the 
