From a broad biological outlook parasitism is a negative reaction to the struggle for existence , and always implies the discovery and adoption of a mode of life that is nearer the line of least resistance. On this view the most diagnostic feature in parasitism is some retreat from strenuous struggle and independent en deavour. To some extent, to put it more metaphorically, the swimmer becomes a drifter. Thus the larvae of an ichneumon fly, which are hatched out within a caterpillar and proceed to devour it from within, are hardly less predatory than the lion which devours the antelope from without. The larvae have none of the degenerative stigmata of thoroughgoing parasites. Simi larly, while many of the protozoon parasites are sluggish through out a great part of their life, many remaining for a long time within the same cell, it is more difficult to apply the term para site to such organisms as the exceedingly active trypanosomes that cause sleeping sickness and allied diseases. They have their quiescent phases, but much of their life is spent in charging about among the blood-corpus cles at a high velocity. Many of the protozoa are much simpli fied cells, but not the flagellate trypanosomes.
And again, as regards ectopara sites, is not the definition blurred by including such types as the flea? That it is more habitual than a leech in its blood-letting does not make it less of a preda tory animal. Its compressed body may be adaptive to escaping capture as it moves swiftly among the hairs of a mammal's skin, but it has no marks of degeneracy in its adult life. It is on an ecological level quite different from that occupied by such types as mange-mites.
Classification of Parasitic Animals.—Protozoon parasites are illustrated by the entire class of Sporozoa, including the malaria organism (Plasmodium) ; by some rhizopods, such as the Amoebae of man's mouth-cavity and intestine; and by some infusorians, such as the mouthless, ciliated, multinucleate Opalina of the frog's rectum, and predatory forms like trypanosomes.
There are no parasitic sponges. Among Coelentera there are but a few instances; thus the polyp, Polypodium hydriforme, occurs on the ova of the sterlet, Acipenser ruthenus; the medusoid, Cunina (Cunoctantha) parasitica is found in close association with another medusoid, Geryonia proboscidalis, and Cunina octonaria in the bell of Turritopsis. The Mesozoa (di cyemids and orthonectids) are all parasitic.
Among plathelminths, a few turbellarians are parasitic (Graf filla in marine molluscs, Anoplodiurn in or on holothurians) ; the trematodes are all either ectoparasites or endoparasites ; the temnocephalids hang on to freshwater crayfish, crabs, turtles, etc., but do not feed on their hosts; the series has its bathos in the cestodes which are all endoparasitic, both in their larval (bladder worm) and adult (tapeworm) stages. All the nemer teans are free-living, unless two or three found on crustaceans can be called parasites. The aberrant Malacobdella found in the bivalve Cyprina islandica does not seem to do more than capture inswept organisms.
Numerous nematodes are parasitic, others saprophytic, but it is difficult to draw the line. Most of the parasites live in the ali mentary canal and find nourishment in the half-digested food or in the putrefying undigested residue. Many of them are most accurately regarded ac internal saprophytes, and are active agile animals. It is otherwise with the hookworm which sucks blood from the wall of the food-canal, or the gapes-worm which does the same in the windpipe of chickens. Distantly allied to nema todes are the Acanthocephala, e.g., Echinorhynchus, thorough going parasites without mouth or f ood-canal. They are digenic, passing from arthropod to vertebrate ; thus Echinorhynchus gigas of the pig has its larva in the grubs of cockchafers and the like.
Parasitism is very rare among chaetopods, but the mouthless pigmy males of Bonellia and Harningia live within the females. The interesting discodrilids are found on freshwater crustaceans, —Branchiobdella, on the gills of Astacus fluviatilis and the American Bdellodrilus on Cambarus. These two types seem to be genuine Oligochaeta, but they are without setae and possessed of suckers and chitinous jaw-plates. In other words, there is a slight convergence towards leeches, in adaptation to a somewhat similar mode of life. Leeches (Hirudinea) themselves should be regarded as predatory, not as parasitic. The small Myzostomaria, e.g., Myzostoma, which form galls on crinoids, are perhaps off shoots from primitive annelids, degraded in relation to the para sitic mode of life. A few rotifers, e.g., Albertia, are parasitic in or on freshwater oligochaets; Seison occurs on the primitive marine crustacean called Nebalia; Discopus attaches itself to Synapta, and Callidina parasitica to the limbs of the freshwater crustaceans Gammarus and Asellus.
There are no parasitic echinoderms. The habit is common among the lower crustaceans, especially among copepods, many of which are called "fish-lice," e.g., Caligus and Lernaea. Many grades occur, for the association may be temporary or permanent (except in the early larval stages), and is often confined to the females. On the other hand the male may be reduced in size and a parasite on the female, as in Chondracanthus. Allied to the barnacles (Cirripedia) are the peculiar Rhizocephala, e.g., Sac culina, which occurs protruding from the ventral surface of the tail of crabs. It starts life as a free-swimming nauplius; it de velops into a cyprid larva and fixes itself to a young crab at the uncalcified membrane around the base of a large seta. Its tissues dedifferentiate and pass into the crab; eventually, as it approaches maturity, it protrudes on the abdomen. The full-grown sac con sists mainly of hermaphrodite reproductive organs, nourished by numerous absorbent root-like processes which spread through all the tissues of the crab. Sacculina lives for about three years, ar resting the crab's growth, and causing degeneration of the gonads. In the case of male crabs the parasite so alters its host's con stitution that it changes its secondary sexual characters, becom ing more or less female in appearance. In short, Sacculina not only effects "parasitic castration," but partly reverses the sex of the male. There are various related forms, one of which, Sesarmaxenos, occurs on a freshwater crab, Sesarma, in the Andamans. Some of the true barnacles, normally hermaphrodite, have small parasitic, "complementary" males, e.g., Scalpellum villosum; others, e.g., S. regium, are unisexual, but the males are minute and parasitic on the females. Even among the higher crustaceans, parasitic forms occasionally occur, notably the epi carid isopods, e.g., Bopyrus and Entoniscus, which infest other members of the class. The young forms are free-living and male, the adults are parasitic and female; but it seems that while all females pass through a male stage, without genital ducts, those males that become functional never grow up into females. Many epicarids cause parasitic castration of their hosts. Some of them afford good instances of hyper-parasitism, or parasite upon para site. Thus a very common Mediterranean species, Danalia curvata, is parasitic on Sacculina neglecta, which in turn is parasitic on the spider-crab, Inachus mauretanicus. Somewhat similar to the epicarids are the cymothoids, not nearly related, though also in the order Isopoda. They infest the gill-chambers, mouth-cavity and skin of various fishes.