But whilst in Trematoda a digestive sac is invariably present except in the sporocyst larval stage, the Cestodes possess no trace of this organ at any stage of their development.
A, The free-swimming embryo.- B, A sporocyst containing young rediae.
The cilia are lost, the eye-spots disappear, the digestive sac vanishes and the larva becomes a sac or "sporocyst" full of germ-cells.
(Highly magnified; after Ziegler.) F, Portion of a sporocyst containing Bucephali in process of development.
Distomum macrostomum, which occurs in various birds, produces a very curious sporocyst in the body of the snail Succinea putris.
In many cases it appears that only the brilliantly coloured tentacle is pecked off by the bird, and as the snail can easily regenerate a new one, this in turn becomes infected by a fresh branch of the sporocyst ramifying through the snail and thus a new supply of larvae is speedily provided (Heckert).
These cells aggregated in masses become the bodies of another generation of larvae within the sporocyst.
The movements and activity of the redia cause it to burst the wall of the sporocyst.
By a series of changes similar to those by which the primary larva arose from a segmented egg, so do these secondary larvae or "rediae" arise from the germ-cells or germ-balls within the sporocyst.