These segments, or " proglottides," become detached in groups, and if kept moist retain their powers of movement and vitality for a considerable time.
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 scolex is biradially constructed, the proglottides flattened, quadrangular and bilaterally symmetrical.
When this is reached, growth of the proglottides ceases.
As a general rule the ripe proglottides are detached in chains and replaced by others which in their turn become detached, the process being repeated for a year or so until the worm weakens and is cast out.
The Cestodes of Elasmobranch fish offer more convincing examples of independent growth of the proglottides, for these are often set free with only the male organs developed, and each attains twice the size of the parental strobila.
The form of the proglottides is most generally a rhombic or trapezoidal figure.
The replacement of detached mature proglottides at the distal end of the Cestode-body by others is not regeneration, for the replacing set has already developed, and in certain cases they can complete their development quite independently after being detached from the parent.
" It has long been known that proglottides of the same species often exhibit sporadic malformation from the normal shape, and the evidence goes to show that the variation was due to arrested growth or some unusual stress or pressure which, acting upon the young strobila, produced a deformation, and that the proglottides so affected could not regain their normal form.
The first two or three proglottides merely become deformed and produce an appearance known as the pseudo-scolex.
(A and B from Lankester's Treatise on Zoology, part iv., C original.) of proglottides or of eggs which are disseminated along with the faeces of the final host and subsequently eaten by herbivorous or omnivorous mammals, insects, worms, molluscs or fish.
On this view, therefore, at least two asexual generations (embryo and scolex) alternate with a sexual one (proglottides); and in the case of Staphylocystis the cyst contains two asexually produced generations, so that in such forms three stages (embryo, primary scolex-buds, secondary scolices) intervene between the proglottis of a Cestode and that of its offspring.
The tapeworm, Taenia saginata, throws off eleven proglottides a day during its mature stage, and if this rate of increase were maintained for a year the total weight of its progeny would be about 550 grammes.
The broad worm, Dibothriocephalus latus, is similarly estimated to discharge 15 to 20 metres of proglottides, weighing 140 grammes.
Few proglottides are developed.
Echinococcifer echinococcus, a minute form with only three to five proglottides, in dog, wolf, jackal.
Known by the presence of the proglottides or mature segments in the stools.