There are several morphological characteristics that distinguish acanthocephalans from other phyla of parasitic worms.
Except for the absence of the longitudinal fibres the skin of the proboscis resembles that of the body, but the fluid-containing tubules of the proboscis are shut off from those of the body. The canals of the proboscis open into a circular vessel which runs round its base. From the circular canal two sac-like projections called the lemnisci run into the cavity of the body, alongside the proboscis cavity. Each consists of a prolongation of the syncytial material of the proboscis skin, penetrated by canals and sheathed with a muscular coat. They seem to act as reservoirs into which the fluid which is used to keep the proboscis "erect" can withdraw when it is retracted, and from which the fluid can be driven out when it is wished to expand the proboscis.
The central ganglion of the nervous system lies behind the proboscis sheath or septum. It innervates the proboscis and projects two stout trunks posteriorly which supply the body. Each of these trunks is surrounded by muscles, and this nerve-muscle complex is called a retinaculum. In the male at least there is also a genital ganglion. Some scattered Papillae may possibly be sense-organs.
Thorny-headed worms begin their life cycle inside invertebrates that reside in lakes and rivers. Gammarus lacustris, a small crustacean that feeds near ponds and rivers, is one invertebrate that the thorny-headed worm may occupy. This crustacean is depredated by ducks and hides by avoiding light and staying away from the surface. However, when infected by a thorny-headed worm it becomes attracted toward light and surfaces itself. Gammarus lacustris will even go so far as to find a rock or a plant on the surface, clamp its mouth down, and latch on, making it easy prey for the duck.
It is thought that when Gammarus lacustris is infected with a thorny-headed worm, the parasite causes serotonin to be massively expressed. Serotonin is a neurotransmitter involved in emotions and mood. Researchers have found that during mating Gammarus lacustris expresses high levels of serotonin. Also during mating, the male Gammarus lacustris clamps down on the female and holds on for days. Researchers have additionally found that blocking serotonin releases clamping. Another experiment found that serotonin also reduces the photophobic behavior in Gammarus lacustris. Thus, it is thought that the thorny-headed worm physiologically changes the behavior of the Gammarus lacustris in order to enter its final host, the bird.
The Acanthocephala are dioecious. There is a structure called the genital ligament which runs from the posterior end of the proboscis sheath to the posterior end of the body. In the male, two testes lie on either side of this. Each opens in a vas deferens which bears three diverticula or vesiculae seminales. The male also possesses three pairs of cement glands, found behind the testes, which pour their secretions through a duct into the vasa deferentia. These unite and end in a penis which opens posteriorly.
In the female, the ovaries are found, like the testes, as rounded bodies along the ligament. From these masses of ova dehisce into the body cavity and float in its fluid. Here the eggs are fertilized and segment so that the young embryos are formed within their mother's body. The embryos escape into the uterus through the uterine bell, a funnel like opening continuous with the uterus. At the junction of the bell and the uterus there is a second small opening situated dorsally. The bell "swallows" the matured embryos and passes them on into the uterus, and from there, out of the body via the oviduct. Should the bell swallow any of the ova, or even one of the younger embryos, these are passed back into the body cavity through the second, dorsal, opening.
The embryo passes from the body of the female into the alimentary canal of the host and leaves this with the feces.
A curious feature shared by both larva and adult is the large size of many of the cells, e.g. the nerve cells and cells forming the uterine bell. Polyploidy is common, with up to 343n having been recorded in some species. The acanthocephalans lack an excretory system, although some species have been shown to possess flame cells (protonephridia).
The earliest recognisable description of Acanthocephala - a worm with a proboscis armed with hooks - was made by Italian author Francesco Redi (1684). In 1771 Koelreuther proposed the name Acanthocephala. Muller independetly called them Echinorhynchus in 1776. Rudolphi in 1809 formally named them Acanthocephala.
Currently the phylum is divided into four classes - Palaeacanthocephala, Archiacanthocephala, Polyacanthocephala and Eoacanthocephala.
Polymorphus spp. are parasites of seabirds, particularly the Eider Duck (Somateria mollissima). Heavy infections of up to 750 parasites per bird are common, causing ulceration to the gut, disease and seasonal mortality. Recent research has suggested that there is no evidence of pathogenicity of Polymorphus spp. to intermediate crab hosts. The cystacanth stage is long lived and probably remains infective throughout the life of the crab.
The life cycle of Polymorphus spp. normally occurs between sea ducks (e.g. eiders and scoters) and small crabs. Infections found in commercial-sized lobsters in Canada were probably acquired from crabs that form an important dietary item of lobsters. Cystacanths occurring in lobsters can cause economic loss to fishermen. There are no known methods of prevention or control.