In every species the segments develop from the scolex distally and increase in size with the maturation of the contained female genital organs.
One view, the monozoic, regards the whole development as a prolonged metamorphosis; another, the polyzoic view, considers that not only is the Cestode a colony, the proglottides being produced asexually, but that the scolex which buds off these individuals is itself a bud produced by the spherical embryo or onchosphere.
The extraordinary variety of form and complication of structure exhibited by the appendages of the scolex are adaptations to fix FIG.
In accordance with this we can regard the development as an adaptive one and the scolex as invaginated for protective FIG.
A peculiar modification of this type of scolex occurs in the Echinobothridae, in which the axial part of the organ (the rostellum) is elongated and provided with several rows of hooks, whilst the phyllidia have partially fused.
The most remarkable feature of this cystic development is the formation in many genera of several internal buds within a common cyst, each of which forms an independent inverted scolex (Coenurus, Polycercus); or these internal vesicles may bud off a large number of scolices on their external surface (Staphylocystis).
D, the entire bladderworm with scolex everted (drawn from Cysticercus pisiformis, common in the rabbit): a, scolex; b, fore-body; c, hind-body and tail.
This fact gave rise in ancient times to the false idea that the tapeworm originated from the union of these segments; and in modern times it has led to the view that the tapeworm is not a segmented organism (the monozoic view), but is a colony composed of the scolex which arises from the embryo and of the proglottides, which are asexually produced buds that, upon or before attaining their full size and maturity, become separated, grow, and, in some cases, live freely for a time, just as the segments of a strobilating jelly-fish grow, separate and become sexual individuals (the polyzoic view).
The peculiar feature of the larval history of Cestodes is the development in most cases of a cyst or hydatid on the inner wall of which the scolex is formed by invagination.
This elaborate type of scolex appears to be an adaptation to grasp the spiral intestinal valve of sharks and rays.