|Etymology:- From the Greek Acantha a prickle and Kephale a head.|
Characteristics of Acanthocephala:-
1)Bilaterally symmetrical and vermiform.
2)Body has more than two cell layers, tissues and organs.
3)Body cavity is a pseudocoelom.
4)Body possesses no digestive system.
5)Body covered by a syncitial epidermis with a few giant nuclei.
6)Has a nervous system with a ganglion and paired nerves.
7)Has no circulatory or respiratory organs.
8)Reproduction sexual and gonochoristic, with vivparous embryos.
9)Adults parasitic on vertebrates.
10)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, 3 feet. They get their name from their proboscis which possesses several rings of backwardly curving spines which they use to attach themselves to the walls of their hosts 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 again as with many parasites 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 and the first step in the reproductive cycle is copulation, which occurs in the vertebrate hosts guts. Males also have a cement gland which they use to seal the females vagina after copulation. The eggs are fertilised by the sperm within the females 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 and they are released by the female in to the hosts guts where they pass out with the faeces. They then remain dormant until eaten by the secondary, or intermediary host. Once inside the secondary hosts 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 hosts 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.
(Primary host and Adults)--Acanthors--(Secondary host and Acanthellas, then Cystacanths)----(Adults in Primary host, mating, larvae)---Acanthors-- and so on.