Dead and shrunken hydatids are more frequently met with in this country than active ones. The animal may die spontaneously, or be killed by the fluid which fills the vesicles being drawn off. In that case the vesicles shrink, and their contents become converted into a fatty debris, which afterwards may become infiltrated with lime salts. In this way the hydatid mass may be represented by a cyst filled with atheromatous material. This may dry-in, and at last we may have nothing left but a stony or mortarlike mass, in which careful search may still discover calcified heads and hooklets (Fig. 188). Besides the distinctive hooklets, or even without them, there are usually in these old cysts portions of the chitinous membrane or cuticle of the parasite. (See Fig. 189, in which by shrinking of the whole lesion, the stratified chitinous membrane has been thrown into folds. The dead and calcified heads are also shown as opaque bodies.) As this chitinous membrane is very resistant it may be found in the midst of the grumous contents and enable the structure to be recognized. In some cases the diagnosis is made chiefly by flakes of this membrane being found, the search for hooklets being in vain.

Fig. 188. - Hooks from taenia echinococeus. x 350.

Fig. 189. - From a .shrunken hydatid of the liver. To the left is liver tissue, which is separated from the lesion by a broad band of connective tissue. Within this is the stratified membrane of the parasite, variously folded. The black spots are dead and calcified heads of the parasite, x 8.

In regard to the distribution of the echinococcus, it is of pretty frequent occurrence in all known lands, but it is particularly common in Iceland, where the men, living in close companionship with the dogs, are much exposed to infection. It is also very common in Australia and neighbouring colonies.

Bothriocephalus Latus

This is the largest tape-worm which occurs in man. It attains a length of from 16 to 26 feet, and possesses from 3,000 to 4,000 segments, which are mostly much broader than long, although the last ones (see Fig. 191) become longer and narrower so as to assume more of a square shape. The breadth at the widest part is about half an inch. The worm is also thick and heavy.

The head (Fig. 190) is oval, and about the twenty-fifth of an inch in breadth. It is blunt at the extremity, and possesses neither hooklets nor suckers, but fixes itself by means of a slit-like groove on either side of the head (see figure).

The sexual organs, and especially the uterus, occupy the middle part of each segment, where they form a rather prominent knot or rosette (Fig. 192). The uterus is composed of a convoluted tube, which gives the rosette-like appearance just mentioned. The sexual organs open in the middle line near the anterior extremity of the proglottis. The eggs are oval in form, and are covered by a brown shell.

Fig. 190. - Head and portion of body of Bothriocephalus latus. x 8. (Leuckart).

This worm is rarely met with in Britain. It is frequent in Sweden, Russia, Switzerland, and Japan. It is specially in fish-eating districts that it occurs.

The scolex form of the worm long eluded observation. It was known that a six-spined embryo formed in the eggs in the usual way, but the habitat of .the cysticercus was unknown. Braun has finally demonstrated its existence in the pike. In some localities every individual pike contains many scolices. It has also been found in some other fish. It was found in the muscles, sexual organs, liver, spleen, etc., of these fish. Braun proved that it was the scolex of this animal by feeding dogs with it. A tape-worm identical with the bothriocephalus developed.

This worm is of rather frequent occurrence in Switzerland and north-east Europe, but it is not unknown in this country. The frequency of fresh-water lakes in Switzerland explains its common occurrence there, from the use of the fish, which form the hosts of the scolex form. Like the other tape-worms, the mature worm occurs in the small intestines.

Several other small and infrequent tape-worms have been met with in man.

The Taenia nana has been observed in Egypt and Italy. It is very small, scarcely an inch in length, and about the fiftieth of an inch in breadth. It possesses a rostellum with hooklets and four sucking discs.

Taenia Flavopuncta

It has been observed a few times in children in America and Italy. It is about a foot in length, and about one-twentieth of an inch in breadth. The head has been described by Parona as devoid of hooklets.

Taenia Madagascariensis

This form has been found in an island on the Madagascar coast.. It is about three inches long, and the head is not yet known.

Fig. 191. - The Bothriocephalus latus. Natural size. (Leuckaet).

Taenia Cucumerina

This worm is from 7 to 10 inches long. Its head possesses a rostellum with a quadruple circle of hooklets to the number of about 60. These are somewhat like the spines of a bramble bush in shape, and dotted over the protruding rostellum, give a curious appearance. The proglottides reach a breadth of about the twelfth of an inch. It is almost constantly present in dogs and cats, and often in large numbers, especially in dogs. It has been found in several cases in man, and it seems to be not infrequent in children. The scolex form has lately been found in the dog-louse (Trichodectes canis), and it can be readily understood how in the process of licking itself the dog often swallows its host, and becomes itself the host of the strobilus. It may be conveyed to children from the tongue of the dog.

Fig. 192. - Proglottis of Bothriocephalus latus, showing female organs. x 12. (Leuckart).

The lower animals are much affected with tape-worms, either in the adult or scolex form. The dog in particular accommodates a number of forms in the intestine, and each of these has its appropriate intermediate host for the bladder-worm. Besides the Taenia cucumerina, mentioned above, the following parasites of the dog may be mentioned:

Taenia Marginata

This is a tape-worm of large size, its usual length being about 5 feet. It is very like the taenia solium, but smaller, the head possessing a rostellum with 30 to 44 hooklets. The scolex form, or Cysticercus tenuicollis, inhabits swine and the ruminants, and it often develops large vesicles in the peritoneum.

Taenia Serrata

A large tape-worm about a yard long, possessing 34 to 38 hooks. Its intermediate host is the hare or rabbit, where it occurs in the peritoneum as the Cysticercus pisiformis.

Taenia Serialis

This tape-worm is half to three-quarters of a yard long. Its head has 26 to 32 hooks. Its scolex form is chiefly found in the peritoneum of rabbits, where it forms a ccenurus, that is to say, a bladder with many heads inside it (see Fig. 171). In this form it is called Ccenurus serialis.

Taenia Ccenurus

This is like the preceding, but larger. Its head possesses 22 to 32 hooks. The intermediate form is in the brain of sheep (exceptionally in that of the calf, as in case observed by the author), and is a ccenurus - the ccenurus cerebralis. There is a comparatively large vesicle studded internally with heads. The parasite produces prominent nervous symptoms, the disease called 'Gid,' 'sturdy,' etc.

Literature

Fully in Leuckart, Paras, des Menschens, 2nd ed., 1879-1894 (first vol., transl. by Hoyle); Neumann, Parasites of Domestic Animals, transl. by Fleming, 1892; Davaine, Traite des Entozoaires, 1877; Bland Sdtton (Intraperitoneal rupture of echinococcus), Brit. Med. Jour., 1892, i., 1184; Braun, Virch. Arch., xcii., 1883, and Zwischenwirth des breiten Bandwurmes, 1886; Taroett (Hydatids in bone), Guy's Hosp. Rep., 1., 1893.