The Phylum Acanthocephala
Etymology: From the Greek Acantha a prickle, and Kephale a head.
Characteristics of Acanthocephala:
- Bilaterally symmetrical and vermiform.
- Body has more than two cell layers, tissues and organs.
- Body cavity is a pseudocoelom.
- Body possesses no digestive system.
- Body covered by a syncitial epidermis with a few giant nuclei.
- Has a nervous system with a ganglion and paired nerves.
- Has no circulatory or respiratory organs.
- Reproduction sexual and gonochoristic, with vivparous embryos.
- Adults parasitic on vertebrates.
- Larvae live in insects and crustaceans.
Acanthocephala is a medium sized phylum (1,000 species) of usually small and always parasitic (in the guts of vertebrates) worms. Over 1,000 have been found in the gut of a single seal.
Most are less than 25mm or 1 inch long, though some species may attain a length of nearly a metre.
They get the name of Spiny Headed Worms (or sometimes Thorny Headed Worms) from their proboscis, which possesses several rings of backwardly curving spines that they use to attach themselves to the walls of their host’s digestive system. This proboscis can be retracted within the body wall by muscular contraction, but it is extended again by hydraulic pressure.
As is the case with many parasites, acanthocephalans have lost many of their organs and tissues – retaining only those they need to grow and to reproduce.
Thus they have no circulatory, digestive or respiratory organs. Everything just passes in and out through their cuticle. They have a simple nervous system, comprising a single ventral ganglion in the proboscis and a few nerves. They do have reproductive organs though and as with many parasites (such as flukes) a complicated life cycle, involving more than one host.
Acanthocephalans come in male and female and (unusually for invertebrates) the males have a penis and the females have a vagina. The first step in the reproductive cycle is copulation, which occurs in the vertebrate host’s guts. Males also have a cement gland, which they use to seal the female’s vagina after copulation. The eggs are fertilised by the sperm within the female’s body and embryonic development occurs their as well.
After a certain stage of development, the larvae become encapsulated and these ‘shelled’ or ‘encased’ larvae are called Acanthors. They are released by the female into the host’s guts, where they pass out with the faeces. They then remain dormant until eaten by the secondary – or intermediary – host.
Once inside the secondary host’s guts, they hatch into a second stage larvae called an Acanthella (as which they grow).
After reaching their full size, they encyst themselves in their secondary host’s tissues where they remain dormant until the secondary host is eaten by the primary host. This encysted stage is called a Cystacanth.
Many acanthocephalan primary hosts are carnivores, or at least omnivores, as this helps ensure the fulfilling of the life cycle by increasing the chances of the secondary host being eaten by the primary host. Some species manage to change the habits of their secondary host, to make it more likely that it is eaten – by making it stay out in the open for instance.