Life-histories of the Digenea.—The classification of Trematodes according to their life-histories, expressed in the divisions Monogenea and Digenea, though a very useful one, breaks down entirely in the case of certain forms. Thus the life-history of Gyrodactylus is probably digenetic rather than monogenetic. Aspidogaster conchicola,[[77]] which lives in the pericardial cavity of the fresh-water mussel (possibly the only case of a Trematode becoming normally mature in an Invertebrate host, since other species of Aspidogaster live in Chelonia), produces larvae which enter another Anodonta and develop directly into the sexual form. In other words, Aspidogaster, though structurally a digenetic form, possesses a life-history which is direct and simple, i.e. monogenetic.
The Holostomatidae, which live in birds of prey and aquatic birds, give rise to eggs from which a minute larva escapes. The fate of this aquatic larva is not directly known, but in all probability after entering a host (Fish, Amphibian, Mollusc), it undergoes a gradual change into what has long been known as a Tetracotyle, from the frequent presence of four (sometimes only three) adhering organs. Fig. 31 exhibits a species which is abundant in the lens and vitreous humour of the eye of the Perch. Its further history is not known, but presumably the Perch is presently devoured by the final host in which the Diplostomum attains maturity. Thus the Holostomatidae are "metastatic" (Leuckart), their (probably) direct development requiring the presence of two hosts.[[78]]
The other Digenea, the life-histories of which are known, belong to the Distomatidae and Amphistomatidae, and we may distinguish the steps by which the complex life-history of the liver-fluke (Distomum hepaticum) has been brought about, by a consideration of that of Distomum macrostomum.
Fig. 31.—Diplostomum (Tetracotyle) volvens. (After v. Nordmann.) × 130. cv, Contractile excretory vesicle; d, intestine; e, calcareous bodies in excretory tubules; ex.o, excretory aperture; gl, glandular adhesive body; ms, oral sucker; ph, pharynx; vs, ventral sucker.
Distomum macrostomum.—This form occurs in the intestine of several common Passerine birds. It is remarkable not only for the large oral sucker, but also on account of the position of the common genital pore at the hinder, and not as usual, at the anterior, end of the body (Fig. 32, A). The eggs pass out through this pore, and are discharged with the bird's excrement. Should a certain snail (Succinea putris) happen to rasp off the epidermis of a leaf upon which the faeces have fallen, the eggs are swallowed and a minute active larva is set free (Fig. 32, B). This penetrates through the thin wall of the digestive tract of the snail, and passing into the connective tissue, throws off its cilia and assumes the shape of Fig. 32, C. This sporocyst, as the larva is now termed, grows rapidly in all directions (Fig. 32, D) at the expense of the snail's tissues, until it becomes impossible to separate parasite and host completely.
Fig. 32.—Life-history of Distomum macrostomum Rud. A, Immature Distomum (really a tailless Cercaria) found in the swollen terminal parts of Leucochloridium (Fig. 33, B) and enclosed in two protective membranes, × 40; B, larva which hatches out of the egg of D. macrostomum, × 125; C, the metamorphosed larva (sporocyst) fourteen days after having entered Succinea putris, and pierced through its intestinal wall; D, actively growing sporocyst. (After Heckert.) go, Genital aperture; int, intestine; ms, mouth sucker; n, nervous system; ov, ovary; ps, ventral sucker; te, testis.
Those branches which lie superficially in the cephalic region of the snail become greatly swollen, cylindrical, and contractile. They are banded with green and white, ornamented with red terminal spots, and pulsate rapidly. Hence these fertile branches of the sporocyst (which in this condition was known as Leucochloridium paradoxum, Fig. 33, B) naturally attract the attention of insectivorous birds, which peck off the tentacles of the snail, and with it the swollen sporocyst-branch. A sphincter muscle closes the cut end of the fertile sac when the bird's bill nips it off. The sac contains large numbers of young D. macrostomum (Fig. 32, A), produced by the division of embryonic cells of the larva (Fig. 32, B), which are apparently blastomeres of the egg reserved for this future use. It is a remarkable circumstance that the old bird itself is immune from infection, and if it swallows these young Distomes, they are digested. Should, however, the snail's tentacle and its contents be offered as food to the nestlings, their weaker digestive powers merely set the Distomes free from the protective membranes (Fig. 32, A), and thus the Blackcaps, Sparrows, and other birds infested by D. macrostomum have acquired the parasite when they were nestlings by the unintentional agency of their parents.[[79]] The snail regenerates its lost tentacles only for the sporocyst to again bud off fertile branches into them.
