With few exceptions they are composed (i) of a minute organ of fixation (the scolex), which marks the proximal attached end of the body; (2) of a narrow neck from which (3) a number of segments varying from three to several thousands are budded off distally.
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 body, or " strobila," consists of a usually minute organ of attachment (scolex or its representative) which is imbedded in the intestinal membrane, and of a series of segments that arise from the base of the scolex and increase in size distally.
The scolex is biradially constructed, the proglottides flattened, quadrangular and bilaterally symmetrical.
The scolex is usually a conical muscular structure.
- Scolex of Calyptobothrium riggii from the Torpedo, magnified to show the four " phyllidia," each of which has a sucker.
These complex organs have apparently arisen by the increase in depth and differentiation of an accessory sucker such as is borne on the phyllidia of the former group. Lastly, the scolex of the more familiar Taeniidae (Tetracotylea) carries a rostellum encircled with hooks and four cup-shaped suckers the margins of which do not project beyond the surface of the body.
The extraordinary variety of form and complication of structure exhibited by the appendages of the scolex are adaptations to fix FIG.
In every species the segments develop from the scolex distally and increase in size with the maturation of the contained female genital organs.
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.
This elaborate type of scolex appears to be an adaptation to grasp the spiral intestinal valve of sharks and rays.
The concentration of nervous matter and ganglionic substance at the oral end of Trematodes is equivalent to the " brain " of the Planarians, but the similar thickening in the scolex of Cestodes is by no means so certainly to be called by that name.
Those Cestodes which possess no very distinct organ of attachment (such, for example, as Gyrocotyle) have no distinct ganglionic thickening more pronounced at one end of the body than at the other; and as these are forms which have retained more primitive features than the rest, and show closer affinity to the Trematodes, it seems highly probable that the complicated nervous thickening found in the scolex, and often compared with the " brain " of other Platyelmia, is a structure sui generis developed within the limits of the sub-class.
This statement is quite consistent with the continuous production of new segments at the neck of the scolex, for such a process is analogous to the development of the segments in a Chaetopod, which is a perfectly distinct phenomenon from the regeneration of new segments to supply the place of a head or tail-end or some other portion that has been lesioned.
Moreover, injury to the scolex, or amputation of that organ, reveals the concomitant absence of a regulative mechanism such as that which generally controls the form and fitness of regenerated organs.
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.
In this way bladders as large as an orange and containing secondary bladders, A each with a scolex, may arise from a single embryo.
In most other cases the tail is not distinguishable, and the body of the larva is separable only into a scolex invaginated with a bladder (= hind-body and tail).
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).
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.
In accordance with this we can regard the development as an adaptive one and the scolex as invaginated for protective FIG.
With regard to the adult worm we have to remember that its two extremities, scolex and terminal proglottis, are different from the intervening region.
Scolex with two " bothria," or modification thereof, usually devoid of hooks.
Scolex with four outgrowths forming organs of adhesion and probably also of locomotion.
The metacestode-larva occurs free in the intestine of fish, Cephalopods and crabs, and is known as Scolex polymorphus.
Scolex with a long head-stalk armed with several rows of hooklets.
Scolex with four complex eversible proboscides.
Scolex with four suckers, rarely hooked, and with a rostellum.