16 Dr. J. Anderson on the Anatomy of Sacculina. 
contact with any other structure. In a large specimen of Saccu- 
lina carcini, 1 found two small mussels living in and attached to 
the inner surface of its ovigerous sac. The ova (fig. 6) are 
enclosed in the sac, and are arranged in a racemose manner, 
enveloped in a very delicate membrane. 
The internal ovaries (figs. 2, 8, & 8’, h), are situated posteriorly 
to the peduncle. They constitute an oval, flattened, pulpy mass, 
dividing the cavity of the parasite into two compartments ; they 
are attached by the centre of their posterior margin to the left 
wall of the ovario-branchial orifice ; and also, as previously men- 
tioned, by the septum which runs along the left side of the sac 
they are connected to the corium (fig. 2g). In the many speci- 
mens examined, I have always found a small tubercle (figs. 2 
& 30), with a minute and apparently horny speck on its sum- 
mit, placed on both surfaces (figs. 3 & 3/, 0) of the body of the 
ovaries lying opposite to one another, a little to the left of the 
centre of its posterior margin. On removing the little speck of 
horny matter, a depres.ivn is seen in the centre of the tubercle, 
apparently communicating with the substance of the ovaries. 
May uot these structures be the orifices of the oviducts, closed 
by a temporary secretion of horny matter till the brood of ova 
in the ovigerous sac has attained its full development? This 
view of the nature of these tubercles has suggested itself to me 
from the difficulty of accounting for the passage of the ova into 
the ovigerous sac. The fact that the ova found on the external 
surface of the ovaries are always more fully developed than those 
further removed from the surface suggests another view of the 
subject—viz. that they are developed in successive layers, and 
thus constitute the ovigerous lamelle. 
Situated immediately posterior to the peduncle, is a well+ 
defined cavity, lined by a special membrane, and containing two 
oval-shaped bodies (2) placed side by side. These pellucid sacs 
(figs. 7 & 8) contain in their cavities peculiarly shaped bodies (c), 
and are provided with convoluted ducts (a). The ducts appear 
to pass towards the nght side of the parasite (figs. 3 & 3’, h) 
along the anterior margin of the ovaries, where they unite and 
become lost. From the close relation of their ducts to the ova- 
ries, it has all along appeared to me that these vesicles probably 
play an important part in the generative economy of the animal. 
This opmion seems to be strengthened by the fact that, on one 
occasion, when examining under the microscope a portion of the 
ovaries in the immediate neighbourhood of the vesicles, I detected 
among their convolutions a tube identical in appearance with 
the structure of their ducts. As yet, I have found it impossible, 
from the soft nature of the tissues of these parasites, to trace the 
ducts to their final termination ; but, from the appearance of the 
