308 Proceedings of the Royal Physical Society. 
tough, brownish-coloured, corrugated, and highly contrac- 
tile structure. It is chiefly by means of this membrane 
that the parasite is attached to the crab on which it lives. 
The part which is attached to the crab forms a short ped- 
uncle (&), but afterwards it suddenly expands to form the 
external sac. The pedunculated portion (6) 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 (h). — The external skin of this structure is a 
continuation of the external sac (a), and contains within it 
a prolongation of the parasite (k), 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 absorbing the required 
nutriment. 
The ovario-hranchial orifice (c) is so named from the two- 
fold function it is supposed to fulfil in the economy of the 
parasite. If a living Sacculiiia is carefully watched for a 
few minutes, this orifice will be seen to open and contract 
slowly, while a current of water may be seen to pass into 
and out of the cavity of the body, the sac at the same time 
alternately distending and contracting. The ova, when 
fully developed, are extruded by this orifice, the structure 
of which confines the water to the sac which contains them. 
The orifice is situated upon the posterior margin of the 
body, and is slightly raised above the level of the sac. 
There is a constriction at its base, and a thickened por- 
tion of the sac plays the part of a sphincter muscle. The 
inner margin of the orifice is thrown into folds, usually 
eleven in number, sometimes of a delicate and pellucid ap- 
pearance. By this arrangement the orifice is capable of 
great distention. 
Thecorium (fig. 2 e). — On reflexion of the external skin we 
