Orders: Cestodes and Acanthocephali.
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.
Archigetes and Caryophyllaeus are the only Cestodes that become fully differentiated in an invertebrate host.
Dorsal surface, but in those Cestodes which possess marginal gonopores this distinction of surface is obscured.
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.
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.
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.
The chief difference between the reproductive organs of the two classes is the presence in Cestodes of a separate vagina and uterus, each of which opens in some families to the exterior by an independent pore.
The vagina of Cestodes is undoubtedly comparable with the so-called " uterus " of Trematodes, but the nature of the Cestode uterus is not so clear.
It has been compared with the canal of Laurer of Trematodes (the vitello-intestinal duct of the ectoparasitic flukes), but if we take the more primitive Cestodes, and especially Aniphilina, into consideration we find that they possess, in addition to the uterus, an anterior vagina (usually present in Cestodes) and a posterior one.
The life-history of Cestodes consists of larval and adult stages, which are usually passed through in different hosts.
In the Cestodes the vagina is present (V); the canal of Laurer (LC) is now vestigial (present in Caryophyllaeus as the posterior vagina).
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.
The eggs of Cestodes consist of oval or spherical shells,(r in.
Such is the general history of Cestodes whose intermediate host is an Invertebrate.
- With regard to the vexed questions of the morphological nature and of the affinities of the Cestodes, divergent views are still held.
The Effects of Cestodes on their Hosts (Shipley and Fearnsides .
The fact of this increased leucocytic activity during the early stages, or the whole course of infection by Cestodes, is indirect proof that these parasites do normally discharge toxic substances into their hosts.
The Planarians are free-living animals, the Trematodes are parasitic upon and within animals, and the Cestodes are wholly endoparasitic.
Within this muscular tube lies a parenchymatous tissue which may be uniform (Cestodes) or differentiated into a central or digestive, and a peripheral portion (some Turbellaria), or finally the central portion becomes tubular and forms the digestive sac (Trematodes), while the peripheral portion is separated from it by a space lined in some forms by a flattened epithelium (most Planarians).
The Cestodes, however, are connected by no such intermediate forms with the other Platyelmia.
From these are given off at irregular intervals short lateral branches, each of which terminates in a flame-cell (f) precisely similar in structure to the flame-cells found in Planarians, Trematodes and Cestodes; here as there the question whether they are open to the body cavity or not must probably be answered in the negative.
Table of Cestodes found in Man Literature.
- (I) Leuckart, The Parasites of Man (Edinburgh, 1886); (2) Braun, The Animal Parasites of Man (London, 1906); (3) Id., " Cestodes " in Braun's Klassen u.