On the Anatomy 0/ Sacculina. 



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expose the underlying coriurn, which invests nearly the 

 whole inner surface of the sac. I have succeeded, in one or 

 two instances, in separating this membrane into two well- 

 marked layers. The external layer is a very thin membrane 

 investing the whole inner surface of the sac, attached at its 

 anterior extremity to the horny ring of the peduncle, and 

 posteriorly to the ovario-branchial orifice. I think it pro- 

 bable, when the external skin is moulted, that its place is 

 supplied by this structure. The inner layer, following it 

 from the ovario-branchial orifice to which it is attached, 

 passes forwards, closely applied to the outer layer, till it 

 nearly reaches the anterior margin of the sac, where it be- 

 comes reflected on to the anterior portion of the peduncle, 

 and can be traced no farther as a separate structure. At 

 the left margin of the peduncle the corium is attached by a 

 septum (fig. 3 g) to a pulpy body embraced in the folds of 

 the ovigerous lamellae. 



Organs of reproduction. — On opening Sacculina by an inci- 

 sion extending through the sac and continued from the 

 ovario-branchial orifice to the peduncle, we expose a pellucid 

 sac (fig. 4 i) filled with ova. This sac is found on both sides 

 of the pulpy body above referred to (fig. 2 A), which it em- 

 braces within its folds. The sac is merely a temporary 

 structure including the ova till their full development ; and 

 at this period I have seen the ovario-branchial orifice plug- 

 ged up by the extruded sac and its contents, and in other 

 cases I have found it lying quite loose in the general cavity 

 of the parasite. In specimens like these, a delicate mem- 

 brane may be separated, by gentle manipulation, from the 

 inner surface of the corium and from the surface of the 

 pulpy body or internal ovaries. This membrane appears to 

 be an ovigerous sac, in the process of growth, destined to 

 receive a brood of ova, but, after their development, to be 

 cast off like its predecessor (fig. 5). The ovigerous sac ap- 

 pears to be continually present in one stage or another of 

 its development, so that the water which passes in at the 

 ovario-branchial orifice is never in contact with any other 

 structure. In a large specimen of Sacculina carcini, I found 

 two small mussels living in and attached to the inner sur- 



