Unisegmental or spuriously (?) segmented Vermes, devoid of organs of special sense and of a digestive tract. There are organs of adhesion in the shape of chitinoid hooks, suckers or grooves. The nervous system is well-developed, and has the form of two or more lateral cords with anterior ganglia. The coelome is represented by irregular channels, the excretory system by longitudinal canals, or by a network of vessels both alike furnished with terminal ciliated funnels, and opening externally by one or more pores or a pulsatile vesicle. Hermaphrodite. Male and female genital organs are rarely present in a single set, but are usually repeated many times. Endo-parasitic.

The sexual worm inhabits the digestive tract of some Vertebrate host, with the exception of Archigetes, which becomes sexual in the coelome of Tubifexrivulorum (Oligochaeta). It possesses but one set of male and female genital organs, and is therefore obviously unisegmental in Archigetes, Caryophyllaeus and Amphilinidae. In other instances it has many sets of genitalia metamerically disposed, and the body of the worm is then usually segmented to correspond. This segmentation, however, is scarcely indicated in some instances (Triaenophorus, Ligula), and it is generally well marked in the anterior region, where the rudiments of sexual organs are not to be traced, and it may even appear without their development. It commences anteriorly behind the head and neck or scolex (infra), and the segments increase in size, etc, the further they are from this spot. The terms 'joint' or 'proglottis' are applied to the segments which are by many authorities regarded as zooids produced by serial gemmation from the scolex and the sexual worm or 'strobila,' therefore, as a compound or colonial organism.

These views are probably incorrect (see pp. 231-2), and it is possible that every Cestode possesses a certain fixed limit of size and growth1.

The non-sexual worm rarely occurs in the digestive tract, as e. g. of Lophius, but as a rule in the flesh, or one of the viscera (liver, brain, etc.) of a Vertebrate. It has been found also in the coelome or tissues of some Mollusca, Arthropoda, and Vermes. It is generally considered to consist of two generations united together: (1) the proscolex, produced by the growth of the embryo, and (2) the scolex, developed from a disc of cells in the body-walls of the proscolex. The proscolex varies much in size: it is usually globular but sometimes oval or band-like, and may be either solid or, as in most Taeniadae, hollow. The scolex consists of a head with its various organs of adhesion and a longer or shorter neck. It is attached to the proscolex, and generally invaginated into it, the position in which it is nearly invariably developed. The scolex may acquire a set of generative organs, and remain permanently in connection with the proscolex, as in Archigetes; it may be set free and lead a wandering life, or enter an intermediate host, as in some species of Tetrarhynchus; and when it is transferred to the alimentary canal of its final host, either the region of the neck may lengthen, sets of generative organs appear, as well as a more or less distinct division into joints, and with the liberation of the first formed joints the proscolex may also be set free, as in Ligula and fish tapeworms, etc.; or else, as most commonly occurs, the proscolex is digested in the stomach of the host and the scolex, passing on into its intestine, affixes itself, the neck lengthens, generative organs and joints appear.

The view that proscolex and scolex are parts merely of a non-sexual worm which becomes sexual on transference to a new host is more probable than that there is an Alternation of Generations, including either three individuals, the non-sexual (1) proscolex and (2) scolex, with (3) the sexual joints; or two, (1) the nonsexual proscolex, and (2) the scolex, which becomes sexual in the second host; see pp. 232-3

1The scolex of Anthobothrium Musteli and Phyllobothrium Lactuca appears to increase 4-6 times in size after it enters its second host. The detached joints may similarly increase immensely, e.g. the strobila of Calliobothrium Eschrichti is 4-5 mm. in length, the free joint 8-9 mm., and the ovum, including its filaments, measures 6-7 mm. Phyllobothrium thridax, Anthobothrium cornucopia, and Tetrarhynchus minutus may also be instanced, but the contrast between the strobila and the joint is not so extreme.

The scolex is rarely devoid of all organs of adhesion, as in Caryo-phyllaeus, Leuckartia, and Abothrium Gadi. Suckers are usually present. There is a single terminal sucker resembling the oral sucker of a Distome in the Arnphilinidae; there are two lateral cephalic grooves in all Pseudo-phyllidea which are produced in Solenophorus and Duthiersia into cornucopia-like structures with basal perforations. Echinobothrium has two flattened suckers, but in other Cestoda there are four, cup-like in Taeniadae and Tetrarhynchus; large, extremely mobile, and crisped marginally in some Phyllobothriums, e. g. Phyllobothrium lactuca; areolate as in Calliobothrium, or pedunculate as in Anthobothrium; whilst Polypoce-phalus, from the intestines of an East Indian Rhinobatis, has the anterior extremity surrounded by sixteen hollow (?) muscular tentacles, as well as armed with two or four suckers. Chitinoid hooks are present in some Taeniadae, in Tetrarhynchidae, Phyllacanthinae, Echinobothrium, and Triaenophorus. They are disposed in one or more circles round the base of a rostellum or central projection from the head in Taeniadae; in numbers upon four contractile proboscides, with which the head is furnished in the Tetra-rhynchidae; and on the upper part of the suckers in the Phyllacanthians and Echinobothrium. The last-named is remarkable for also possessing three longitudinal series of hooks on each surface of the neck.

The hooks are curved, e. g. Taeniadae; forked, e. g. Acanthobothrium; straight, Echinobothrium; implanted by large and forked roots in the Taeniadae, and moved by special muscles2.