RHIZOCEPHALA. 227 



We thus see that, by changes in the nucleus, the most important 

 parts of the body of the adult Sacculina begin to be formed ; viz., 

 the inner visceral sac, the brood-cavity, and the mantle-fold which 

 surrounds it. The visceral sac is not completely encircled by the 

 brood-cavity, for, at the point of attachment of the sac, the inner 

 mantle-layer passes over into the perivisceral ectodermal layer ; this 

 transition point is the so-called mesentery, which DELAGB assumes to 

 have a ventral position in the body. 



Having now obtained a general idea of the development of the form of the 

 body in Sacculina, we must add a few words as to the rise of its most important 

 organs. In the mantle region the changes are not great. In the late stages, 

 the mantle-cavity breaks through into the perivisceral cavity, and thus arises 

 the cloacal aperture (Fig. 109, cl), which lies almost opposite the stalk of the 

 Sacculina, but somewhat to the left side of the body. While the ectodermal 

 cells of the mantle lengthen to form transverse connective fibres (Fig. 109 B}, 

 the mesoderm-cells change into longitudinal muscle-strands and the sphincter of 

 the cloaca. 



More important alterations occur in the region of the visceral sac. The 

 ganglion (</) here forms by an immigration of ectoderm-cells, in which not only 

 the perivisceral ectodermal layer, but also the inner mantle-layer (by means 

 of the mesenterial margin ?) are said to take part. Whereas, in earlier stages, 

 the whole inner space of the visceral sac was almost exclusively occupied by 

 the ovarian rudiment, numerous mesenchy me -cells now wander from the stalk 

 into the perivisceral sac, surround the ovary, forming a peritoneal envelope 

 around it, and fill the space between the body- wall, the ganglion, and the ovary. 

 The ovarian rudiment simultaneously breaks up into two lateral lobes connected 

 by a commissure. The manner in which the short oviduct arises is not quite 

 clear, but DELAGE believes that he can trace it back to a paired lateral ecto- 

 dermal invagination. This latter, widening inwardly, gives rise to the so-called 

 atria (a), on whose walls the glands of the ovarian sac (cement glands, dr) 

 appear as lateral outgrowths. The vasa deferentia arise in the same way 

 through ectodermal invaginations near the stalk of the visceral sac, while the 

 actual testes () are derived from mesoderm-cells which become attached to the 

 ends of these ducts. 



After the Sacculina, completely enclosed within the central tumour 

 {in the perivisceral cavity), has in almost every respect attained the 

 .grade of development of the adult, it rises to the surface of the central 

 tumour, passing out through the widened opening of the perivisceral 

 cavity (Fig. 109 A, /). The fold by which this cavity was formed 

 now draws back to the base of the stalk and soon entirely disappears. 

 An increase in size takes place in the Sacculina after leaving the 

 central tumour, and this causes constant pressure on the ventral wall 

 of the abdomen of the host (Fig. 109 B, L) leading to gangrene of 

 the part thus affected, and to the consequent formation of an opening 

 through which the body of the Sacculina passes out freely, its stalk 



