Page:EB1911 - Volume 26.djvu/436

 

 TAPEWORMS. The Cestodes or Tapeworms form a class of purely endoparasitic Platyelmia, characterized by their elongate shape, segmented bodies, and the absence of a digestive system. With few exceptions they are composed (1) 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. 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). Whether this view is soundly based is discussed below; the fact remains, however, that a tapeworm is, with few and rare exceptions, not directly comparable at all points with a liver-fluke or indeed with any other organism. The influence of parasitism has so profoundly influenced its structure that its affinities are obscured by the development of specialized and adaptive features.

In contrast to these segmented or “merozoic” Cestodes, a few primitive forms have preserved a unisegmental character and form the Monozoa or Cestodaria. We may therefore divide Cestodes into the Monozoa and the Merozoa.

Order I.—

This order comprises a few heterogeneous forms which probably constitute at least three families.

Family I. Amphilinidae.—Oval or leaf-shaped animals found in the sturgeon and certain other fish.

Amphilina foliacea (fig. 1) is in many ways closely allied to the Trematoda, from which, however, it is distinguished by the want of a digestive system. One end of the body (usually designated anterior) is provided with a glandular pit (fig. 1, Aa) which is regarded as a sucker or as related to the uterine opening (birth-pore). The excretory system consists of peculiar cells, each of which bears several “flames” or bunches of synchronously vibrating cilia. These cells are imbedded in the peripheral parenchyma, and lead into convoluted excretory tubes that form an anastomosis opening to the exterior by a pore at the “hinder” end of the body. The epidermis consists of pyriform cells, which send richly branched processes to the superficial cuticle. The parenchyma is made up of stellate cells the processes of which form a reticulum. The reproductive organs consist of the parts shown in fig. 1, A, and it will be seen that, in addition to the openings of the male

