The figures 1 to 5 are intended to show diagrammatically the life-history of one of the typical Cestoda, genus Taenia. Figure 1 represents the sexual animal as it is found in the segmented form, called 'strobila' from the analogy of a fir-cone, in the intestinal canal of a carnivorous or omnivorous vertebrate 'host,' such as the dog, or human subject. Figure 2 represents the distal half of a segment, or 'proglottis' as it may be seen before the great accumulation of ova in the uterus has caused the disappearance of the other sexual organs, male and female, which each segment contains. Figure 3 represents a ripe segment, so distended with embryos as to have caused the ripe proglottides, which retain considerable locomotor powers, to be called 'ovaria ambulantia.' Figure 4 shows one of the microscopic embryos, the so-called 'proscolex' as it appears when set free from its shell within the stomach, into which it is introduced. Figure 5 shows the cystic stage or scolex into which such a proscolex as that shown in Figure 4 developes, when it belongs to Taenia coenurus, which differs from most other Tapeworms in having its proscolex proliferating as drawn in the figure, instead of producing a solitary 'new head' or 'scolex.' This cystic stage is passed in the tissue of some solid organ, such as the liver or the muscles; but in the particular case of Taenia coenurus, most usually in the brain of the sheep, though sometimes in other parts of the body of this ruminant, as also of rodents.

Cestoda: Hydroidea.

Plate XIV. Cestoda: Hydroidea.

FIG. 1. Tapeworm, as found in the intestinal canal of man or of a dog, semi-diagrammatic; after Van Beneden, Memoire sur les Vers Intestinaux, Paris, 1858, PL xxvi. Fig. 25.

THE 'head' or 'nurse' so-called is armed with a circlet of spines, as is the case with Taeniae which are harboured in the intestines of birds and of carnivorous mammals; whilst the Taeniae of Amphibians and herbivorous mammals are not possessed of this armature. Posteriorly to the circlet of spines is seen a circlet of four suckers. An unsegmented neck follows the suckers. The first segments are small, but as the distance from the head increases, so their size and development increase. A Tapeworm is not a colony composed of an asexual head and sexual proglottides or segments. The segmentation of the body is probably acquired, and not primitive as generally supposed.

FIG. 2. Posterior half of an unripe segment of Taenia coenurus, to show the generative organs, male and female; after Leuckart, Die Menschlichen Parasiten, ed. 2, p. 399, Fig. 165.

a. Water-vascular or excretory canal. Two vessels, a dorsal and ventral, one of which is much larger than the other, and is not shown here, run along either side of each segment, parallel with and close to each other, and are connected with their fellows on the opposite side of each segment by a transverse anastomosis. This transverse connecting vessel takes in the last segment the shape of a median vesicle into which the lateral vessels converge, and through which they open to the exterior. In the larger Taeniae, e. g. T. solium, the dorsal longitudinal vessels are aborted.

b. Uterus, in the unripe segment running as a straight tube from the posterior part of each segment to the anterior. In some Taeniae it is transverse. Where the genital pores are double (right and left), the uterus remains single, e. g. in T. elliptica.

c. The albumen gland or vitellarium. In the small Taeniae the gland is saccular, in the larger tubular, and the tubes branched and anastomosing. They contain small nucleate cells which break down into a tenacious clear fluid. The gland is homologous with the bilaterally symmetrical yolk gland or vitellarium of other Cestoda. Its duct joins the 'fertilising'canal at the spot where the latter is surrounded by a mass of unicellular glands which make up the shell gland indicated in this figure by a slight expansion above the albumen gland.

d. Germ-glands, one on either side, composed of tubes much branched in the larger Taeniae. They contain clear uninucleate ova or germs. The ducts unite in the middle line, and the common canal thus formed unites with the fertilising canal which connects the spermatheca to the uterus, before it is surrounded by the shell-gland.

e. Testes appended in a racemose manner by very delicate ducts to the vasa deferentia. They occupy the 'dorsal' surface of the segment, whereas the female organs are confined to the 'ventral' surface, and were more abundant in the anterior half of the segment which is removed, than in this, the posterior.

f. Intromittent organ, essentially a specialization of the muscular ductus ejaculatorius. It is armed with spines in some Cestoda, which favour its retention in the vagina of its own segment. It is here figured as retracted and coiled up spirally.

g. Vagina dilating into an oval receptaculum seminis or spermatheca, before joining the duct of the two germ-glands. It opens externally in the posterior half of a saucer-like depression, the porus genitalis, on one side of a segment, in the anterior half of which the male outlet is situated. The generative outlets are similarly arranged in Taenia elliptica, a Tapeworm commonly found in the dog and cat; but the glandular organs being double, the outlets are double also, and exist on both lateral edges in each segment. In Taenia mediocanellata from the human species, the common generative depression is situated some way behind the middle of the lateral border of each segment; in Taenia solium it is nearer to the middle line, whilst in Bothriocephalus latus, which, like the two Tapeworms named, infests the human subject, both sexual orifices are situated on the ventral aspect of the segment. The statement that the male orifice may occur in some cases on the edge, and the female on the surface of a segment appears to be erroneous.