Parasites: A Treatise on the Entozoa of Man and Animals / Including Some Account of the Ectozoa

by T. Spencer Cobbold

Some other filariform nematodes have been imperfectly described. Of these, Gescheidt’s Filaria oculi canini (F. trispinulosa, Diesing) was probably a sexually-immature worm, and the same may be said of the encysted worms found by Mr Mather in the mucous coat of the intestines and in the liver ducts and acini (Filaria hepatica, Cobbold). Of more interest is Leisering’s hæmatozoon (Strongylus subulatus). These minute worms occupy the veins, the largest females not exceeding 1/12″ in length. They are viviparous, and thus form another source of embryonic hæmatozoa. A single drop of infected venous blood commonly carries from four to six mature worms. In this place may be mentioned Dr Osler’s Strongylus canis bronchialis. The largest males measure 1/6″ and the females fully 1/4″. In the worms sent to me by Prof. Osler I saw no evidence of strongyloid structure, and in his description he avoids all mention of the presence of any caudal hood in the male. I regard the worms as Filariæ (F. Osleri, Cobbold). Very great interest attaches to them from the fact that they produce a destructive canine epizoöty, resembling the ordinary “husk” or parasitic bronchitis of calves, lambs, and other domesticated animals. The only other genuine strongyle known to infest the dog is Eustrongylus gigas. This is a very common parasite in wolves. I have already spoken of this parasite at some length (Book I, p. [207]), and can only further refer to the recently published case by Mégnin (quoted below, and at full length in my paper in the ‘Veterinarian’ for April, 1879). The Museum of the Royal Veterinary College contains three fine examples of this worm coiled within the kidney of a dog, or rather within the renal capsule, for the substance of the organ is almost entirely wanting. These are from Bickford’s case.

Amongst the many good “finds” made by Lewis in India, not the least interesting is that appertaining to Cheiracanthus robustus. Lewis, indeed, supposed that he had detected Echinorhynchi in chestnut-sized tumours of the walls of the stomach, but, as I pointed out at the time, the parasites were entirely destitute of Acanthocephalous structure. As is well known, this curious nematode infests various felines, such as the wild cat, puma, and tiger. In addition to the above canine nematodes we have the wrinkled threadworm (Trichosoma plica) infesting the bladder. This is of much more frequent occurrence in the fox. In the list Trichina spiralis must also be included, although, so far as I am aware, it has only been seen in dogs that have been subjected to feeding experiments.

Another nematode common to the fox and dogs, and infesting the cæcum, is the whipworm (Trichocephalus depressiusculus). It is very rare in the dog. Lastly, there is the important little strongyloid worm generally known as Dochmius trigonocephalus. At the hands of Leuckart the general structure and development of this entozoon have received complete elucidation. It infests the small intestines, and is found alike in the fox, wolf, and cheetah. It has also been obtained from Canis lagopus and C. azaræ. Dr Krabbe did not encounter this worm in Icelandic dogs; nevertheless, he obtained it in a blue fox which died in Kjoerbolling’s menagerie, and which had come from Iceland. In Danish dogs it occurred in less than 2 per cent. The embryos of this worm are rhabditiform and possess three long bristle-like teeth, the slender tail being furnished with a distinct appendage at the tip. They develop in moist situations, where they feed freely, grow rapidly, and change their skins, throwing off the caudal tip with the first month. It seems evident that they do not require a change of hosts, since Leuckart succeeded in rearing the sexually-mature Dochmii by introducing the rhabditiform larvæ into the stomach of the dog; moreover, his experiments upon water-snails belonging to the genus Physa gave negative results.

Of Arachnidan parasites (Trachearia) infesting the dog, by far the most interesting is the well-known Pentastoma tænioides, shown by Leuckart to be the adult condition of the still better known Pentastoma denticulatum. It resides in the nasal sinuses. As already mentioned in a former part of this work, these parasites present four marked stages of growth, namely (1), the embryo, (2) the pupa, (3) the active larva (P. denticulatum), and (4) the sexually-mature worm. As the eggs and their embryonic contents are lodged in the nasal mucus of the dog, and are commonly distributed by the act of sneezing on the part of the animal, the sources of infection are not far to seek. Clearly the larvæ usually get introduced to the bodies of mankind and herbivorous animals by the ingestion of unclean vegetable matter. The embryos set free in the stomach bore their way to the liver and other viscera, in which organs encystation and moulting subsequently take place. The fondling of dogs infested by pentastomes may prove dangerous by a more direct transference of the eggs to the hands and mouth. As regards the dog, the adult parasite has been known to prove fatal. A very striking instance of this kind was recorded by Prof. Dick, where the worms wandered into the trachea producing asphyxia.

The ectozoa of the dog, though not numerous as species, are of importance in relation to mange. The follicle-mites form a family by themselves (Demodicidæ), and, as already observed, those infesting the dog and cat are, alike, mere varieties of the human species (Demodex folliculorum, var. caninus and var. cati). Whilst the human parasite restricts itself to the face, the canine variety (fig. [52]) will occupy any part of the dog’s body. The follicle-mite of the cat, however, usually confines itself to the ear. According to Mégnin, to whose beautiful monograph we owe so much, two or three dozen of these parasites may be found occupying a single follicle of the dog. Acne-like pustules are thus formed, and when they are very numerous death may result from the excessive irritation, which is usually accompanied with depilation. An interesting example of this kind recently occurred at the Royal Veterinary College. In regard to the ordinary mange-mite (Sarcoptes canis, Gerlach) M. Mégnin points out that it is in all respects identical with the human itch-insect. In the wolf and fox, however, the same species forms well-marked varieties (S. scabiei, var. lupi and var. vulpis). As regards true insect parasites and tormentors of the dog, I can only allude to a few of them. In tropical America dogs are said to be attacked by the larvæ of a species of gadfly (Œstrus canis), whilst in Africa they are often fatally bitten by the tsetse (Glossina morsitans). In addition to the flies (Diptera), several kinds of fleas (Aphaniptera) frequently prove troublesome (Pulex canis, P. martis, and P. penetrans), and the same may be said of certain lice (Hemiptera). The common louse of the dog (Trichodectes latus) proves especially noxious to young puppies. Of the two other species, namely, Hæmatopinus piliferus and H. canis, the former is tolerably common, whilst the latter is comparatively rare. This species is also found on the ferret. A new form of mite (Chorioptes ecaudatus), infesting the ears of the ferret, has recently been described by M. Mégnin.

I have already referred to several of the parasites of the cat-tribe (Felidæ), but some others require notice. Only two flukes (Amphistoma truncatum and Hemistoma cordatum) have been described as infesting the cat. Of the tapeworms, Tænia crassicollis is the best known. This is derived from Cysticercus fasciolaris of the mouse and rat. It is not uncommon to find this scolex in the sexually-immature tænioid state in the liver, measuring six or seven inches in length. An exceedingly interesting communication by Dr Romano, of Gemona (Frioul), demonstrates the possibility of severe feline epizoöty as due to this entozoon. As I gather from an account given in the journal quoted below, “during the summer of 1876, Dr Romano was informed by his confrère Dr Leoncini, a physician practising at Osoppo, that for about a fortnight most of the cats in a certain hamlet of the town had died without appreciable cause after presenting the following symptoms:—Gradual wasting, with complete loss of appetite, retracted abdomen, slight diarrhœa at first, then constipation, abundant saliva, contraction of the elevating muscles of the upper lip in some subjects, great prostration of strength, loss of the visual faculty. Some of the feline patients no longer heard or appeared no longer to hear their master’s voice; some vomited and seemed to experience relief, for the appetite improved, but they soon died like the others. Nervous phenomena, epileptiform convulsions, and more frequently colic, also showed themselves. Having visited the locality (of the outbreak), Dr Romano could not at first procure any corpses for the post-mortem examination, for the children had thrown them into the Tagliamento, which flows at the foot of the fortress of Osoppo. It was only after the lapse of some days that he was able to open one of the animals which had just succumbed. The principal evils were remarked in the stomach, the walls of which were retracted and formed the seat of a catarrhal inflammation, from the products of which a long, white, flat worm was removed with care for examination. All the other organs were in good condition. The examination of the helminth in the stomach, made with the help of Dr Leoncini and Fachini, showed that the flat worm (white, and with the body divided into rings, 12 centimètres long, and 5 or 6 millimètres broad) had all the characters of the tænias, and this was confirmed by a microscopic examination of the head. A few days later Dr Romano made an autopsy of two other cats. In one of the corpses he noted the alterations described above, and found a tænia smaller than the first; in the other the same lesions without any helminth. This negative circumstance very naturally disconcerted Dr Romano, but several people of the place came to assure him that they had seen their cats, during the course of the malady, after violent and repeated efforts at vomiting, throw up a sort of white cord, which they recognised as corresponding with the tænia he showed them. Thus confirmed and reassured in his diagnosis, Dr Romano sought to identify the species.” In this connection it is specially interesting to note that “during the whole summer the inhabitants of Osoppo had been over-run by bands of rats proceeding from the fortress. They were combated by means of cats, and it was the best hunters among the felines that succumbed. Here was, therefore, a striking relation of cause and effect which could not be gainsayed.” Dr Romano communicated his observations to the National and Royal Veterinary Society, but by an error in the report the species appears to have been described as Tænia tenuicollis instead of T. crassicollis. In this connection I have only further to add that the wild cat is infested by a tapeworm scarcely an inch in length (Tænia lineata). A species of Bothriocephalus (B. decipiens) likewise infests the domestic cat, in common with most of the wild felines, such as the tiger, puma, ounce, and jaguar. Dr Bancroft brought me a specimen from an Australian cat. The nematodes of the cats are very abundant. Dr Bellingham found a trichosome (T. felis cati) in the urinary bladder of the wild cat. This is probably identical with T. plica. A tolerably common nematode is Dochmius tubæformis, which occurs not only in the cat but also in the leopard, puma, jaguar, ounce, panther, and also in Felis tigrina and F. mellivora. In the last-named and in other Brazilian felines an echinorhynchus (E. campanulatus) was found by Natterer. A strongyle (Sclerostoma dispar) infests the lungs of the puma. A species of spiroptera (S. subæqualis, Molin) infests the œsophagus and stomach of the lion and tiger. Redi also noticed a species of Filaria beneath the skin of the lion. Physaloptera terdentata (Molin) and P. digitata (Schneider) infest the stomach of the puma. I can only allude also to Tænia laticollis of the lynx, Pentastoma recurvatum, occupying the frontal sinuses and air-passages of the ounce, and Ligula reptans the subcutaneous tissues of the leopard. Lastly, there is the Olulanus tricuspis of the domestic cat. I have examined the lungs of three cats containing this parasite, which was first described as an entirely new species by Leuckart. The adult worms, only 1/25″ in length, occupy the walls of the stomach. Thence they are apt to migrate or stray into the lungs and liver, where they encyst themselves. When myriads of them are thus encysted a kind of nematode tuberculosis is set up. This disease I have elsewhere called olulaniasis. Ordinarily, however, the encysted condition of olulanus is to be found in the muscles of mice, which are thus said to be olulanised. Clearly, as Leuckart’s experiments substantially prove, the domestic cat acquires the adult worm by catching and devouring olulanised rodents. Every now and then the disorder thus created produces a virulent and fatal feline epizoöty.

Bibliography (No. 45).—(Anonymous), “Curious Note on two Tænia from the Dog (asserting that neither strong whisky nor boiling water would kill them),” ‘Med. Commentaries,’ vol. xvi, p. 370, 1791.—Araujo, A. J. P. S., “A Filaria immitis e F. sanguinolenta no Brazil,” ‘Gazeta Medica da Bahia,’ Julho de 1878.—Baillet, C., “On Dochmius, and on a Worm found in the Heart and Vessels of a Dog,” from ‘Journ. Vet. du Midi,’ in the ‘Veterinarian,’ p. 549, 1862.—Baird, W., “Note on the Spiroptera sanguinolenta found in the Heart of Dogs in China,” ‘Proc. Linn. Soc.,’ vol. ix, Zool. Div., p. 296, 1867.—Balbiani (l. c., Bibl. No. 28).—Beneden (see Van Beneden, below).—Bickford (l. c., Bibl. No. 28).—Chapman, “On Ascaris mystax (leptoptera) in the Tiger and American Wild Cat,” ‘Proc. Acad. Philad.,’ 1875, pp. 14 and 17.—Cherry, E. F., “Worms in the Stomach of a Dog,” in the ‘Farrier and Naturalist,’ 1829, vol. ii, p. 303.—Clamorgan, J. de, “Serpents dans les reins,” in his ‘La Chasse du Loup,’ 1570; see also Bibl. No. 28.—Cobbold, “On the Prevalence of Entozoa in the Dog, in relation to Public Health,” ‘Journ. Linn. Soc.,’ vol. ix (Zool. Sect., No. 37, p. 281), 1867; also in ‘Lancet,’ April 27, 1867, p. 521; and in supp. to ‘Entozoa,’ 1869.—Idem, “Observ. on Entozoa, with experiments in regard to Tænia serrata and T. cucumerina,” ‘Linn. Trans.,’ 1858.—Idem, “On some new Forms of Entozoa,” ‘Linn. Trans.,’ 1859.—Idem, “Further Observ. on Entozoa, with Experiments,” ‘Linn. Trans.,’ 1861.—Idem, “On Filaria immitis,” ‘Proc. Zool. Soc. of Lond.,’ Nov. 18, 1873, p. 736.—Idem, “Observ. on Hæmatozoa,” in the ‘Veterinarian,’ Oct., 1873.—Idem, “Parasites of the Dog,” in ‘Manual,’ 1874, l. c., chap. ix, p. 86.—Idem, “Description of a new generic type of Entozoon (Acanthocheilonema) from the Aard Wolf (Proteles),” ‘Proc. Zool. Soc. of Lond.,’ Jan. 13, 1870.—Idem, “On Diatoma compactum,” in ‘Linn. Trans.,’ vol. xxii, p. 363, ‘Proc. Zool. Soc.,’ March, 1861.—Idem, various letters on “Canine Epidemics in relation to Lumbricoid Worms in Dogs,” in the ‘Field,’ Dec., 1872.—Idem, “Remarks on the Life-epochs (biotomes) of T. cœnurus and T. echinococcus,” in a paper on ‘Animal Individuality,’ ‘Journ. Linn. Soc.,’ Zool. Div., vol. viii, p. 163.—Idem, “Worms in the Heart (Mr. M’Innes’ specimen from Charleston),” the ‘Veterinarian,’ Feb., 1875.—Idem, “Note of Lewis’ Discoveries,” in ‘Nature,’ March 11, 1875, vol. ii, p. 363.—Idem, “Remarks on Eustrongylus gigas” in the ‘Veterinarian,’ April, 1879.—Coles, E. C., “Worms in the Heart and Œsophagus of a Dog,” ‘Path. Soc. Rep.,’ March, 1878.—Dare, J. J., “Death of Dogs from Worms in the Heart,” the ‘Field,’ Feb. 24, 1872.—Davaine, ‘Traité,’ l. c., 2nd edit., p. 290 (for refs. to lit. of Strongylus gigas).—Delafond (with Grube), “Note on a Verminiferous kind of Blood of a Dog, caused by Hæmatozoa of the genus Filaria,” from ‘Ann. de Chimie et de Physique,’ in ‘Ann. of Nat. Hist.,’ vol. xi, 1843, and ‘Comp. Rend.,’ in ‘Lond. Phys. Journ.,’ p. 28, 1843; also from ‘Gaz. Med.,’ in ‘Med.-Chir. Rev.,’ vol. xxxv, p. 393, 1843–44; see also ‘Edin. New. Phil. Journ.,’ vol. lii, p. 233, 1852, and ‘Veterinarian,’ vol. xviii, p. 216, 1845.—Dick, “Worms the cause of Sudden Death in a Dog,” the ‘Veterinarian,’ vol. xii, p. 42, 1840 (Pentastomes).—Diesing, ‘Syst. Helm.,’ vol. ii, p. 327 (Eustrongylus).—Ercolani, G. B., “Osservazione elmintologiche sulla dimorfobiosi nei Nematodi, sulla F. immitis e sopra una nuova specie di Distoma dei cani,” ‘Mem. Accad. Bologn.,’ v, 1874–5.—Frank (see Bibl. No. [28]).—Gay, J., Specimen of Olulanus in Hunterian Museum, and marked in the ‘Catalogue’ “No. 1814A, Lung of Cat, pneumonic, from the presence of parasites.”—Gay, “Nematodes of Panther,” in ‘Hering’s Repert.,’ 1873, from ‘Il. Med. Vet.’—Herbst, “On Trichinæ in the Badger,” from ‘Ann. des Sci. Nat.,’ in ‘Assoc. Med. Journ.,’ 1853, vol. i, p. 491.—Hoysted (and Sir J. Fayrer), “On Filaria sanguinis (really F. immitis),” ‘Lancet,’ March 1, 1879, p. 317.—Jamieson (see Manson).—Jardine (see Manson).—Klein, J. T., “Worms found in the Kidneys of Wolves,” ‘Phil. Trans.,’ vol. xxxvi, 1729.—Krabbe, “Husdyrenes Indvoldsorme,” ‘Tidsskrift for Veterinairer,’ 1872.—Leblanc, “Subcutaneous Tumour in the Dog, due to the presence of Strongylus gigas,” from ‘Recueil de Méd. Vét.,’ in ‘Edin. Vet. Rev.,’ Dec., 1863; see also Bibl. No. 28.—Idem (see Mégnin).—Legros, “Hæmatozoa of Dogs,” in ‘Rec. de Méd. Vét.,’ i, p. 947.—Leisering, “On Hæm. subulatum,” in ‘Virch. Archiv,’ 1865.—Leuckart (see Bibl. Nos. [28], [29]).—Idem, (for development of T. serrata, &c.), ‘Die Blasenbandwürmer und ihre Entwicklung,’ Giessen, 1856.—Idem, ‘Bau und Entwicklungsgeschichte der Pentastomen (especially P. tænioides and P. dent.),’ Leipzig, 1860.—Idem, ‘In relation to the genus Dochmius,’ see his standard work, l. c., s. 433, and also ‘Arch. f. Heilkunde,’ Bd. ii, s. 212 (see also Parona).—Lewis, T. R., “On Nematoid Hæmatozoa of the Dog,” ‘Quart. Journ. Micr. Soc.,’ 1875, vol. xv.—Idem, “On Filaria sanguinolenta, Echinorhynchus, &c., from the Dog,” in his oft-quoted ‘Memoir,’ Calcutta, 1874.—Linstow, O. von, “Ueber die Muskulatur, Haut und Seitenfelder von Filaroides mustelarum, v. Ben.,” ‘Arch. f. Naturg.,’ xl, s. 135 (with figs.).—Idem, “Einige neue Nematoden,” &c., ‘Arch. f. Naturg.,’ p. 293, Bd. xxxix.—Manson, P., “On F. sanguinolenta and F. immitis,” in his ‘Report on Hæmatozoa,’ in the ‘Customs Gaz.,’ No. xxxiii (with numerous figs.), Jan.–March, Shanghai, 1877. (N.B.—The same journal, Rep. No. 12, Gaz. No. xxx, contains remarks on worms in the heart of dogs by Dr A. Jamieson. The subject is also discussed by Dr Jardine.) See also ‘Med. Times and Gaz.,’ Oct. 20, 1877, p. 480.—Mather, T., “Filariæ found in the Intestines of a Dog,” ‘Veterinarian,’ vol. xvi, p. 434, 1843.—McConnell, “On D. conjunctum,” ‘Lancet,’ March 30, 1878, p. 476.—Mégnin, P. “On Myobia,” ‘Abhandl. ueber eine neue Gruppe von Acariden (u. s. w.),’ in ‘Revue f. Thierheilkunde,’ Oct., 1878, p. 149.—Idem, “Sur un parasite des oreilles chez le furet,” ‘Rec. de Méd. Vét.,’ Oct., 1878.—Idem (avec C. Leblanc), “Note sur le Cysticercus cellulosæ developpé chez le chien,” ‘Bullet. de la Soc. cent. vét. de Paris,’ 1873.—Idem, “Sur le Strongylus gigas,” Bullet. de la Société Entom. de France, No. 3, 1879.—Melnikow, N., “On Tænia cucumerina,” in ‘Arch. f. Naturg.,’ 1869, p. 62.—Miller, E., ‘Six Cases of Strongyle in the Kidney of Putorius vison’ (quoted by Davaine from Museum of Boston, U.S.).—M’Innes (see Cobbold).—Molin, ‘Nuovi myzelmintha,’ s. 34, 1859.—Idem, ‘Una monog. del gen. Physaloptera (P. tordentata),’ 1860, s. 17.—Idem, ‘Una monog. del gen. Spiroptera (S. subæqualis),’ 1860, s. 13.—Nettleship, E., “Notes on the Rearing of Tænia echinococcus in the Dog from Hydatids, with some observations on the anatomy of the adult worm,” ‘Proc. Roy. Soc.,’ 1866, No. 86, p. 224, with figs.—Noseda, B., “Six Strongyles in the Kidney of the Agouara-gouazura or Puma,” in Don F. de Azara’s ‘Quadrupèdes du Paraguay,’ Paris, 1801 (see also Azara, Bibl. No. 28).—Osborne, T. C., “Worms found in the Heart and Blood-vessels of a Dog (with symptoms of hydrophobia),” ‘Western Med. Journ.,’ rep. in ‘Bost. Med. and Surg. Journ.,’ vol. xxxvii, p. 448, 1847–48.—Osler, W., “Verminous Bronchitis in Dogs (from Strongyles),” ‘Veterinarian,’ June, 1877, p. 387.—Owen, “On the Anatomy of Linguatala tænioides,” ‘Trans. Zool. Soc.,’ vol. i, 1835, and in ‘Lond. Med. Gaz.,’ 1835.—Idem, “Anatomical description of two species of Entozoa, from the Stomach of a Tiger, one of which forms a new genus, Gnathostoma,” ‘Proc. Zool. Soc.,’ part iv, 1836.—Parona (and Grassi), “On a new Species of Dochmius (D. balsami),” ‘Reale Instituto Lombardo di Scienze e Lettere, Rendiconti,’ vol. x, fasc. vi, 1877.—Raynold, T. M., “Tapeworm in the Pointer and Spaniel,” ‘Veterinarian,’ vol. xiv, p. 694, 1841.—Romano, “Epizoöty amongst Cats from Tænia crassicollis,” ‘Giornale di med. vet. practica,’ Août, 1877, and in ‘Journ. de Méd. Vét.,’ Avril, 1878.—Schuppert, M., “Mechanical Obstruction of the Heart (of a Dog) by Entozoa, causing Death,” ‘New Orleans Med. News and Hosp. Gaz.,’ Jan., 1858, also ‘Bost. Med. and Surg. Journ.,’ vol. lvii, 1857–58, and in ‘Med.-Chir. Rev.,’ 1858.—Stirling, “On the Changes produced in the Lungs by the Embryos of Olulanus tricuspis,” ‘Quart. Journ. Micr. Sci.,’ 1877, p. 145.—Van Beneden, “On the Transmigration of the Entozoa” (in answer to MM. Pouchet and Verrier), from the French, by Busk, in ‘Quart. Journ. Micr. Sci.,’ 1862.—Idem, “Researches on the Intestinal Worms,” from Van der Hoeven’s abstr. in the ‘Nederlandsch Tijdschrift voor Geneeskunde,’ in ‘Ann. Nat. Hist.,’ vol. iii, 3rd ser., 1859.—Idem, “New Obs. on the Development of the Intestinal Worms,” from ‘Compt. Rend.,’ in ‘Ann. Nat. Hist.,’ vol. xiii, 2nd ser., 1854.—Idem, “On Filaroides mustelarum,” ‘Mém. sur les vers intest.,’ 1858, p. 267.—Welch, “On Filaria immitis,” ‘Lancet,’ March 8, 1873; also in ‘Month. Micr. Journ.,’ Oct., 1873, p. 157 (with three plates).—Wilson, W., “A Parasitic Worm infesting the Air Sinuses of the Weasel,” ‘Midland Naturalist,’ May, 1878.—Wright, “On Worms found in the Pulmonary Artery of a Dog,” ‘Lancet,’ 1845, and in ‘Veterinarian,’ vol. xviii, p. 52, 1845.—Youatt, “On Tumours (containing Entozoa) in the Stomach of a young Tiger,” ‘Veterinarian,’ vol. x, p. 619, 1837; see also Owen on ‘Gnathostoma.’—Idem, “Worms (Ascaris marginata) in the Nose of a Dog,” ‘Veterinarian,’ vol. v, p. 337, 1832.

PART V (Pinnipedia).

Following the order of classification adopted in my description of the Mammalia in the ‘Museum of Natural History,’ I proceed to speak of the internal parasites of the seals (Phocidæ) and walruses (Trichecidæ). From their piscivorous habits one would naturally expect the seals to be largely infested with entozoa, and yet, though sufficiently victimised, they are not liable to entertain so great a variety of helminths as the fishes themselves on which they feed.