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 (b) 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 (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-branchial orifice (c) is so named from the two- 

 fold 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 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. 



The corium (fig. 2 e). — On reflexion of the external skin we 



