14 Dr, J, Anderson on the Anatomy of Sacculina. 
composed of two joints: the second and third pairs (d! d"’) are 
larger than the first, and are both biramous. The rami are fur- 
nished with bristles. The under surface of the body is prolonged 
into two spines (¢ e), which project beyond the posterior margin 
of the carapace. Besides these terminal spines, I have observed, 
through the transparent body, two other structures (ff), which 
resemble very much the middle pair of spines described by 
Darwin as occurring on the larva of Chthamalus stellatus. The 
greater portion of the body is occupied by an oval mass of nearly 
spherical globules (gy). The various transformations of the larva 
remain yet to be determined. 
The Adult Animal. PI. I. figs. 10, 11, 12, 15. 
According to the present state of our knowledge, this parasite 
seems to be peculiar to the Deecapod Crustacea. All the speci- 
mens I have obtained have been attached over the terminal por- 
tion of the intestinal canal (figs. 10 & 110’) of the crabs on 
which they were parasitic, the females of which they appear to 
infest more than the males. This latter circumstance seems to 
be owing to the large size of the purse of the female, as compared 
with that of the male crab, affording them a better protection 
and means of support. 
The external sac (figs. 2, 4.a).—The external skin is a tough, 
brownish-coloured, corrugated, and highly contractile structure. 
It is chiefly by means of this membrane that the parasite is at- 
tached to the crab on which it lives. The part which is attached 
to the crab forms a short peduncle (0), but afterwards it suddenly 
expands to form the external sac, The pedunculated portion (d) 
is very firmly connected by means of a horny ring (d) to the 
skin which invests the gut of the crab. The posterior extremity 
of the sac remains open, forming a small orifice (c), which I 
have called ovario-branchial. 
The peduncle (b)—The external skin of this structure is a 
continuation of the external sac (a), and contains within it a 
prolongation of the parasite (/), which passes through the horny 
ring, and rests upon the intestine of the crab. The prolonged 
portion is tubular. In this arrangement we have evidence for 
the parasitic nature of Sacculina, and are entitled to regard the 
anterior portion of the peduncle as the mouth. The mouth, 
structurally as such, is entirely absent; and the only way the 
animal appears to derive its nourishment is by this process ab- 
sorbing the required nutriment. 
The ovario-branchial orifice (c) is so named from the twofold 
function it is supposed to fulfil in the economy of the parasite. 
If a living Sacculina is carefully watched for a few-minutes, this 
orifice will be seen to open and contract slowly, while a current 
