4f)0 
O N I S C U S. 
8. Onifcus aquaticus, the round-tailed onifcus: tail 
rounded, with a forked ftyle on each fide; antennas four. 
In this fpecies the lengthening of the legs from the front 
to the back part is gradual, the difference not being fo 
great nor fo fudden as in the preceding fpecies. It is a 
native of the clearer kind of ftagnant waters. In the fe¬ 
male the ovary is very diftinguiffiable, in the form of a 
large inflated valve beneath the body. This fpecies is vi¬ 
viparous, and of a very prolific nature. See fig. 7. 
9. Onifcus marinus: femi-cylindrical; tail oval-oblong. 
It inhabits the European feas, and is fuppofed by Dr. 
Wm. Elford Leach to be a variety of the O. entomon. 
10. Onifcus linearis: body linear; tail four-toothed. 
It inhabits the European and Indian leas. 
11. Onifcus chelipes: feet chelate, like the claws of a 
crab ; fegments of the body ten. This is a large fpecies; 
and inhabits the Atlantic Ocean, among fea-weed. See 
fig- 8. 
12. Onifcus bicaudatus : femi-cylindrical, with two 
tails as long as the body. It inhabits the feas of Norway. 
13. Onifcus fcopulorum : body pale-yellow, with brown 
ftreaks. Inhabits the feas of Norway. 
14. Onifcus Americanus : abdomen covered with twelve 
fcales ; hind-legs long and rufous; tail rounded. Inha¬ 
bits the American Ocean. 
15. Onifcus pfora : abdomen naked beneath; tail femi- 
oval, acute. It inhabits Norway. This is Ihown at fig. 9. 
and lying on its back at fig. 10. 
16. Onifcus phyfodes : abdomen naked beneath; tail 
ovate. Inhabits Europe. This is reprefented, at fig. 7. 
from Dr. Shaw’s General Zoology ; but he gives no de- 
fcription of it. 
17. Onifcus prasguftator. This is defcribed in the 
American Tranf. before referred to, by Benj. Henry La- 
trobe, F.A. P. S. It nearly refembles the preceding fpe¬ 
cies. It is called by filhermen the lunfe; is about two 
inches long, and is always found holding itfelf firmly by 
its fourteen legs to the palate of an animal to which Mr. 
Latrobe gives the name of Clupea tyrannus. It is with 
difficulty (fays the author) that the infedt can be fepa- 
rated, and perhaps never without injury to the jaws of 
the fifli. The filhermen therefore confider, the infedt as 
efi'ential to the life of the fiffi ; for, when it is taken out, 
and the fiffi is thrown again into the water, he is inca¬ 
pable of fwimming, and foon dies. I endeavoured, in 
numerous inffances, to preferve both the infedt and the 
filh from injury, but was always obliged either to deftroy 
the one or to injure the other. I have fometimes fuc- 
ceeded in taking out the infedt in a brilk and lively ftate. 
As foon as hewasfet free from my grafp, he immediately 
fcrambled nimbly back into the mouth of the fiffi, and re¬ 
fumed his pofition. In every inftance he was difguftingly 
corpulent, and unpleafant to handle ; and itfeemed that, 
whether he have obtained his poll by force or by favour, 
whether he be a mere traveller or a conftaht refident, or 
what elfe may be his buiinefs where he is found, he cer¬ 
tainly has a fat place of it, and fares fumptuoufiy every 
day. 
18. Onifcus thoracicus : body ovate, inequilateral, with 
about fifteen indiftindt joints, indented at the fides, the 
fix pofterior (hoofing into long lateral fafciculate fleffiy 
ramous appendages, and the extremity furniffied with fix 
Ample recurved ones, two of which are larger than the 
reft; antennae four, fliort, the outer pair longeft, and 
only vifible above; the two firft joints of the body fur¬ 
niffied with along, flat, oar-like, flelhy, fin, or cirrus, on 
each fide ; the other joints with fimilar ffiort ones ; legs 
fourteen; very fliort, crooked, and concealed beneath; 
the abdominal valves are large, cover the whole under 
part of the body, and form a receptacle for the ova. 
Length, including the pofterior appendages, fcarcely half 
an inch. Colour ufually orange; lateral appendages 
v. hitilh. The male is very inferior in fixe, of a more (len¬ 
der form, and deftitute of the cirri on the anterior part of 
the body ; and thofe on the pofterior joints are Ample, 
not branched, as in the female; in other refpedts they 
agree. 
This is alfo a parafitical infedt. It inhabits the thoracic 
plate of the Cancer fubterraneus, (Linn. Tranf. vol. ix.) 
concealing itfelf between the flelhy part and the (hell, and 
forming a tumour on one fide. From this fituation it has 
been extracted alive, and kept in that ftate for fever?! 
days in a glafs of fea-water. As this infedt feems to be 
pofiefled of little or no locomotive power, it is probable 
the greater part of the eggs or young mult perifli; for it 
mu ft be in one of thefe (fates that it finds its way under 
the thoracic (hell of the crab, and there receives that nou- 
riffiment which, in all probability, is tue only means of its 
exiftence. The very difproportionate fize of the fexes is 
wifely adapted to an animal whofe habitation is fo con¬ 
fined. This and the three following are from the Linn. 
Tranf. vol. ix. where they are defcribed by Mr. Montagu 
among other “ Marine Animals found on the South Coaft 
of Devonffiire.” The prefent fpecies is reprefented of 
the natural fize at fig. 12. and magnified at fig. 13. 
19. Onifcus fquillarum. This is another parafitical 
fpecies, inhabiting the fame part of the prawn as the pre¬ 
ceding is found to do in the C. fubterraneus. The body 
is inequilaterally ovate, compofed of thirteen flat joints, 
the articulations forming as many fcallops on the fides ; 
the legs however are only fourteen, very fliort, crooked, 
and ufually folded-up and concealed under the (even firft, 
or anterior, fcallops on each fide ; the under-part of the 
body between the legs is covered with broad membranes, 
that collapfe and form a receptacle for the eggs, which are 
extremely fmall and numerous. Length rarely half an 
inch. Colour pale-greeniffi, and glofl'y above; the abdo¬ 
minal membranes dark at their edges. 
The mod incurious cannot but have noticed the tu¬ 
mour fo common on the thorax of the prawn, or ffirimp, 
during the fummer-months, that is occafioned by the 
lodgment of this animal, whofe growth occafions the 
diftortion of the (hell. This tumour forms a fecure afy- 
lum for the protedtion of the more than ufually foft and 
membranaceous bodies of thefe parafitical onifici. “ That 
an infedt fo extremely common, and obvious to the moil 
curfory obferver, (fays Mr. M.) (hould not have found a 
place in the Syftema Naturae, appears very extraordinary ; 
and I have been induced to defcribe it, becaufe it feems 
to have been omitted by even the more modern fyfte- 
matifts; or is certainly mifplaced, and not where it ought 
to be.” The defcription above applies to the female. 
The male, which has hitherto efcaped obfervation, is pro¬ 
bably very minute, as in the preceding fpecies. The diffi- 
milarity in the fides of thefe infedts is occafioned by the 
unequal prefiure they receive from the thoracic (hell of 
the crab ; this inequilateral growth is therefore not con- 
ftantly alike, but depends on the fide of the thorax each 
individual inhabits. 
20. Onifcus teftudo: body fub-ovate, compofed of 
eight joints, riling to a ridge on the back ; the plates ele¬ 
vated at their edges; the four firft fall very low on the 
fides, and obfcure the anterior legs; along each fide of the 
body a row of fmall tubercles ; the front fub-bifid ; an¬ 
tenna: four, very fliort, lower pair hid beneath; eyes pro¬ 
minent, black; pofterior end obtufely pointed; caudal 
fins beneath, obfcure; legs fourteen, fliort and ftrong, the 
three pofterior pairs longeft ; all furniffied with a Ample 
claw. Length two lines only; colour dull-red, with a 
white fpot on the anterior part of the back, but, as the in¬ 
fedt dies, this mark is loft. This rare fpecies is ffiown, in 
two pofitions, magnified, at fig. 14 and 15. 
21. Onifcus gracilis: body long, (lender, fub-com- 
prefled, fub-cylindric, with eight joints, terminated by five 
large caudal appendages truncated at their ends; thefe 
are defledted nearly at right angles with the body, the 
middle one fixed ; lateral ones jointed, and fpread a little 
upwards in a femicircular form ; antennas four, fliort; 
the anterior pair of legs large, with broad feet, and (lightly- 
hooked claws; the reft fliort and (lender.' Length five 
lines. 
