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.
These cells aggregated in masses become the bodies of another generation of larvae within 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.