Occurrence of Cestodes in Domestic Animals.[[103]]—Among domestic animals, the dog is, undoubtedly, the most frequently attacked by Taeniae. Six species of Taenia (T. serrata, marginata, coenurus, echinococcus, krabbei, and possibly T. serialis), Dipylidium caninum (the commonest form), Mesocestoides lineatus, and three or four species of Bothriocephalus have been found in the dog. The table of life-histories (p. [83]) shows that sheep, rabbits and other Rodents serve as the intermediate hosts, in which the cystic stages of the species of Taenia are found. Hence the prevalence of T. serrata in a given locality is connected with the abundance there of the rabbit and hare, in which the larva (Cysticercus pisiformis) occurs. Bothriocephalus cordatus develops from the young stage present in the fish which the Icelanders give to their dogs. In Iceland and certain parts of Australia T. echinococcus infests one-third to one-half the number of dogs examined; a fact connected with the frequency of Echinococcus in man in these countries.
In sheep the most noteworthy and dangerous parasite is Coenurus cerebralis (or the cystic stage of the dog-taenia, T. coenurus), which gives rise to the disease known as "gid" or "staggers." It is found in various parts of the brain or spinal cord, and the symptoms differ according to the position of the parasite. If this presses upon one hemisphere the sheep describes circles and finally falls: if on the optic lobes, the eyes are affected: if the pressure affects the cerebellum the movements of the sheep are uncertain and incoordinated. Four or six weeks after the appearance of the symptoms, death results from cerebral paralysis, or from general debility, and the loss of sheep incurred by this disease (happily less frequent in England than formerly) has been calculated by Youatt at a million for France annually; at 35 per cent of the flocks for England in bad seasons; and about 2 per cent for Germany. Besides sheep, which are most subject to "gid" during their first year, various ruminants—Goat, Ox, Moufflon, Chamois, Roe, Antelope, Reindeer, Dromedary—are attacked in the same way. A similar form, Coenurus serialis Baill., is common in the wild rabbit in this country, and in Australia in the hare and squirrel. It forms large swellings in the connective tissue of various parts of the body, but usually does not affect the health of the host. It is not known in what carnivore Taenia serialis Baill. normally occurs. Experiments have, however, shown that it develops rapidly in dogs.
The preventive measures which are steadily diminishing the prevalence of the Cestode parasites in man in some parts of Western Europe cannot be dealt with here, but it may be noticed that the Jewish observance with regard to swine is the surest preventive measure against taeniasis and trichinosis. Careful inspection of meat and general cleanliness, are the leading measures that in these hygienic matters secure the greatest immunity from disease.
Table of the Life-Histories of the principal Cestodes of Man and the Domestic Animals.
| Cestode. | Final host. | Larva. | Intermediate host. |
| Taenia serrata Goeze | Dog | Cysticercus pisiformis Zed. | Rabbit, Hare, Mice (liver and peritoneum) |
| T. marginata Batsch | Dog, Wolf | Cyst. tenuicollis Rud. | Monkeys, Ruminants, Ungulates (in peritoneum) |
| T. saginata Goeze (= T. mediocanellata Küch.) | Man | Cyst. bovis Cobb. | Ox, Giraffe (in muscles) |
| T. solium Rud. | Man | Cyst. cellulosae Rud. (? Cyst. acanthotrias Weinl.) | Pig, Man, Monkeys, Bear, Dog, Cat, Black Rat (in various organs) |
| T. crassicollis Rud. | Cat and other Felidae, Stoat | Cyst. fasciolaris Rud. | Rat, Mouse, Bat (liver) |
| T. coenurus Küch. | Dog, Arctic Fox | Coenurus cerebralis Rud. | Brain of Sheep, Ox, Goat, Dromedary, Camel, Antelope, Horse |
| T. serialis Baill. | ? Dog | Coenurus serialis Baill. | Rabbit (connective tissue) |
| T. echinococcus v. Sieb. | Dog, Dingo, Jackal, Wolf | Echinococcus polymorphus Dies.(incl. E. multilocularis found in Man) | Man, Monkeys, many Carnivores, Rodents, Ungulates, Ruminants, and Marsupials; also in Turkey and other birds |
| Moniezia expansa Rud. | Sheep, Ox, Goat, etc. | Unknown | |
| Thysanosoma fimbriata Dies. | Sheep, Cervidae | Unknown | |
| Stilesia globipunctata Riv. | Sheep | Unknown | |
| Anoplocephala perfoliata Goeze | Horse | Unknown | |
| Dipylidium caninum L. (= Taenia cucumerina Bloch = T. elliptica Batsch) | Man, Dog, Cat | Cysticercoid larva (Fig. 43), Cryptocystis trichodectis Vill. | Body-cavity of Trichodectes and Pulex of Dog |
| Hymenolepis murina Duj. | Mouse, Rat | Cercocystis Vill.[[104]] (develops in parental host) | Usually absent |
| H. nana v. Sieb. | Man | Unknown | |
| H. diminuta Rud. (= Taenia flavopunctata Weinl.) | Man, Mouse, Rat | Cercocystis Vill. | Meal-moth, Asopia (Pyralis) farinalis; also certain Orthoptera and Coleoptera |
| Drepanidotaenia gracilis Zed. | Duck, Goose, Wild Duck | Cercocystis Vill. | The Ostracods Candona rostrata and Cypris compressa, and also Cyclops viridis |
| D. anatina Krabbe | Duck | Cerc"cystis V"ll. | Cypris incongruens, and also Perch |
| D. setigera Fröh. | Goose | Cerc"cystis V"ll. | Cyclops brevicaudatus |
| D. infundibuliformis Goeze | Common Fowl | Cerc"cystis V"ll. | House-fly |
| Dicranotaenia coronula Duj. | Duck | Cerc"cystis V"ll. | Cypris ovum |
| Davainea proglottina Dav. | Fowl | Cerc"cystis V"ll. | ? Limax cinereus, L. agrestis |
| D. madagascariensis Dav. | Children | Unknown | |
| D. friedbergeri v. Linst. | Pheasant | Unknown | ? Ants |
| Mesocestoides lineatus Goeze | Dog | Unknown | |
| Bothriocephalus latus Brems. | Man, Dog, ? Cat | Plerocercoid, i.e solid, elongate larva, with no bladder | Probably first enters an Invertebrate host, which is eaten by Pike, Perch, Trout, etc. |
Fig. 40.—A, Stickleback (Gasterosteus aculeatus) infested by an advanced larva of Schistocephalus solidus Crepl. B, The larva. All × 1½. (From specimens in the Cambridge University Museum.)
Structure and Development of Cestoda.[[105]]—Of the unsegmented Cestodes, Caryophyllaeus mutabilis, from the intestine of carp and other Cyprinoid fishes, is the most easily accessible form. Triaenophorus nodulosus, which is very useful for the study of the excretory system, occurs mature in the pike. In the body-cavity of the Stickleback (Fig. 40) a large, broad, yellow worm may sometimes be found, the larva of Schistocephalus solidus Crepl., which occurs in the intestine of Terns, Storks, Mergansers, and other birds. Species of Ligula are found in the same birds. The intestine of a Lophius or Cyclopterus ("lump-fish") contains, usually, the early and intermediate stages of various Cestodes, while the alimentary canal of Elasmobranchs often contain many peculiar Tetrarhynchidae and other forms. For the study of development, the Taenia anatina from the duck may be used. The ripe proglottides are collected, and the eggs placed with Cypris ovum in an aquarium, with the probability that some of the embryos will enter the Ostracod, and the peculiar Cysticercoid may be bred.[[106]] Cysticercus pisiformis and Coenurus serialis, which occur commonly in rabbits, are also suitable objects for examination.
A Cestode such as Echinobothrium (Fig. 36) is divisible into head and proglottides. Moniez has suggested that the head is really the morphologically hinder end of the body, in which case the formation of proglottides would closely resemble the mode of segmentation of an Annelid larva. The close similarity, however, between the Cysticercoid larva (Fig. 43, F) and the Cercaria of a liver-fluke, seems to show that the anterior end is the same in both cases, and since it bears the central part of the nervous system, we may reasonably call it the "head." Moreover the hinder end of a Platyhelminth usually possesses the chief excretory pore. Another difficulty is the determination of dorsal and ventral surfaces. Authors are agreed,—on the analogy of Trematodes, in which the testes are usually dorsal and the ovaries ventral,—that the dorsal and ventral aspects of a Cestode are determined by the position of these organs, although the often radially formed "head," the lateral or superficial position of the genital apertures, and the variability of these features, render it a matter of considerable doubt whether "dorsal" and "ventral" are more than useful conventional terms. The suckers and hooks are borne on a muscular cap, the "rostellum," which is only slightly developed in the Ichthyotaeniae. The body is solid, and is divisible into an outer muscular coat—enveloped in a (possibly epidermal) investing membrane—and an inner parenchymatous tissue containing the chief part of the excretory, nervous, and reproductive systems. One or two pairs of longitudinal excretory vessels are present, usually connected by transverse ducts and opening by a single terminal pore. Occasionally a regularly paired arrangement of lateral or secondary pores is present (Figs. 38 and 41, for.sec). Flame-cells occur at the end of the fine tubules (Fig. 38), and the whole system is well developed, but may undergo degenerative changes in the older proglottides. The central nervous system varies according to the degree of differentiation of the rostellum; and, owing to the difficulty of staining the nerves and the contradictory statements of authors, we do not yet possess a fully reliable account of the nervous system even of the commoner Taeniae. Free nerve endings and other sensory terminations have been recently stated to exist in the cuticle of Cestodes and Trematodes. If true, this would tend to show that the parasitic mode of life of these animals demands a complex nervous system comparable with that of the Turbellaria.
