Fig. 100.—Fully formed larva of Echinorhynchus proteus from the body-cavity of Phoxinus laevis. (From Hamann.) Highly magnified. a, Proboscis; b, bulla; c, neck; d, trunk; e, e, lemnisci.
Family I. Echinorhynchidae.—The body is elongated and smooth. The proboscis-sheath has a double wall, and the proboscis is invaginated into it. The central nerve-ganglion lies in the middle line, as a rule on the posterior blind end of the proboscis-sheath. The papillae which bear the hooks are only covered with a chitinous cap at their apex, and the hooks have a process below. This family is by far the largest; a few species only can be mentioned. Echinorhynchus proteus lives in its mature form in fishes; the young forms, up to a centimetre in length, are found living freely in the intestine of numerous fresh-water fishes. Those found in Gobio fluviatilis, the gudgeon; Leuciscus virgo; Lota vulgaris, the burbot or eel-pout; young trout; Thymallus vulgaris, the grayling, seldom surpass this size, but those found in Acerina cernua, the pope fish; in Abramis bipunctatus; in Esox lucius,the pike, and in older trout, attain or surpass double the length. As the parasites grow older they bury their proboscis and neck in the wall of the intestine, the inner surface of which is studded with the orange-coloured bodies of the parasites. The proboscis is so deeply sunk in the wall of the alimentary canal as to form a papilla on its outer surface (Fig. 92). The larvae of E. proteus are found in the body-cavity of Gammarus pulex, one of the Amphipod Crustacea, and also in the same position in numerous fresh-water fishes; they must have passed into this first host by the mouth and alimentary canal. If the liver of an infested minnow, Leuciscus phoxinus, be examined, it will be found to contain on its surface numerous spherical or egg-shaped capsules of an orange colour, 2 to 2.5 mm. in length; these contain the larval forms of the parasite. They develop into the adult form when the first host is eaten by a carnivorous fish, but a complication may take place when the larval form is found in Gammarus, as the latter, the first host, may be eaten by a fish (intermediate host) in which the larva does not become mature, and only develops sexual organs when eaten by a carnivorous fish (second host). The larval form is also found in Nemachilus barbatulus, Gobio fluviatilis, and the sticklebacks Gasterosteus aculeatus and G. pungitius.
E. clavula Duj. is found in Salmo fario, Abramis brama, Cyprinus carpio, Gobius niger, Lepadogaster gouanii, etc.; E. linstowi Ham. in Leuciscus idus, Abramis ballerus, Abramis bipunctatus, and Acipenser huso; E. lutzii Ham. was found by Dr. Lutz in Brazil in the intestine of Bufo agua; E. angustatus Rud. occurs in such numbers in the perch, Perca fluviatilis, as to almost occlude the lumen of the intestine, and one out of every three or four fish in certain districts is infested by it. It is also found in the pike, Esox lucius, and the barbel, Barbus vulgaris. The first or larval host of this species is the Isopod Asellus aquaticus. E. moniliformis Brews. is stated to attain maturity in the human intestine. Except for the fact that G. gigas has once been observed in the same place, this is the only human parasite amongst the Acanthocephala. Its normal second hosts are Mus decumanus and Myoxus quercinus, and its first or larval host, the larvae of the beetle Blaps mucronata. E. porrigens Rud. is found in considerable numbers in the small intestine of a fin-whale (Balaenoptera sibbaldii), and E. strumosus Rud., in the small intestine of a seal (Phoca vitulina), and in the body-cavity of the angler fish (Lophius piscatorius). E. acus is common in the whiting, Gadus merlangus.
Family II. Gigantorhynchidae.—Large forms with ringed, flattened, and Taenia-like bodies. The hook-papillae are covered all over with transparent chitinous sheaths with two root-like processes. The proboscis-sheath is muscular and without a lumen. The central nervous system is excentrically placed below the middle of the so-called sheath. The lemnisci are long twisted tubes with a central canal.
Hamann places three species in this family: Gigantorhynchus echinodiscus, G. spira, and G. taenioides; but as he points out that E. gigas resembles these in its more important structural features, it seems advisable to include it here under the name G. gigas. The members of the first family often present a transversely ringed appearance after death, but the Gigantorhynchidae are ringed when alive, and the circular canals in the skin show a certain regularity, being arranged one between each two rings. There is no lumen in the proboscis-sheath, which is not attached to the boundary between the proboscis and the trunk, but to the inner surface of the proboscis, and the whole can be retracted within the anterior portion of the body, which is invaginable. There are always eight cement-glands, and other differences exist in the musculature, hooks, and position of the nervous system.
G. gigas occurs in the adult state in the small intestine of swine; in Europe its first or larval host is believed to be the grubs of Melolontha vulgaris and Cetonia aurata, but these beetles are absent from America, though the parasite infests American hogs. Stiles[[220]] has recently made some experiments which tend to show that in the United States the source of infection is some species of the beetle Lachnosterna, and he has succeeded in infecting the grub of L. arcuata by feeding it on the eggs of the parasite; from one larva he took 300 parasites six weeks after feeding it. L. arcuata is, like M. vulgaris, phytophagous, but the grubs of both the beetles are fond of frequenting manure heaps and patches of dung, and thus are much exposed to the dangers of infection.
G. echinodiscus inhabits the intestine of ant-eaters, having been found in Myrmecophaga jubata and Cycloturus didactylus. G. spira lives in the king vulture Sarcorhampus papa, and G. taenioides in Dicholophus cristatus, a species of Cariama.
Family III. Neorhynchidae.—Sexual maturity is reached in the larval stage. The proboscis-sheath has a single wall. A few giant nuclei only are found in the sub-cuticle and in the lemnisci. The circular muscle layer is very simply developed. The longitudinal muscle-cells are only present in certain places.
This family includes two species, Neorhynchus clavaeceps and N. agilis, which afford interesting examples of paedogenesis. The sub-cuticle and the lemnisci are dominated by a few giant nuclei, which remain in the embryonic state and do not break up into numerous nuclei as in other forms. The musculature is but little developed and the longitudinal sheath hardly exists. The proboscis-sheath consists of a simple muscular layer, and the short proboscis has few hooks and presents an embryonic appearance.