Editing Acanthocephala (section)

==Morphology==
{{More citations needed section|date=March 2021}}

[[File:Corynosoma drawing.jpg|thumb|left|alt=Diagram of Acanthocephalan morphology|Some key features of acanthocephalan morphology]]

Several morphological characteristics distinguish acanthocephalans from other phyla of parasitic worms.

===Digestion===

Acanthocephalans lack a mouth or [[alimentary canal]]. This is a feature they share with the [[cestoda]] (tapeworms), although the two groups are not closely related. Adult stages live in the [[intestine]]s of their host and uptake nutrients which have been [[digestion|digested]] by the host, directly, through their body surface. The acanthocephalans lack an excretory system, although some species have been shown to possess [[flame cell]]s (protonephridia).

===Proboscis===

[[File:Parasite140083-fig5 Figs 31-36 Cathayacanthus spinitruncatus.tif|thumb|Scanning electron microscopy of proboscis of ''[[Cathayacanthus spinitruncatus]]''<ref>{{cite journal |last1=Amin |first1=O. A |last2=Heckmann |first2=R. A |last3=Ha |first3=N. V. |year=2014 |title=Acanthocephalans from fishes and amphibians in Vietnam, with descriptions of five new species |journal=Parasite |volume=21 |page=53 |doi=10.1051/parasite/2014052 |pmid=25331738 |pmc=4204126}} {{open access}}</ref>]]

The most notable feature of the acanthocephala is the presence of an [[anterior]], protrudable [[proboscis]] that is usually covered with spiny hooks (hence the common name: thorny or spiny headed worm). The proboscis bears rings of recurved hooks arranged in horizontal rows, and it is by means of these hooks that the animal attaches itself to the tissues of its host. The hooks may be of two or three shapes, usually: longer, more slender hooks are arranged along the length of the proboscis, with several rows of more sturdy, shorter nasal hooks around the base of the proboscis. The proboscis is used to pierce the gut wall of the final host, and hold the parasite fast while it completes its life cycle.

Like the body, the proboscis is hollow, and its cavity is separated from the body cavity by a ''septum'' or ''proboscis sheath''. Traversing the cavity of the proboscis are [[muscle]]-strands inserted into the tip of the proboscis at one end and into the septum at the other.  Their contraction causes the proboscis to be invaginated into its cavity. The whole proboscis apparatus can also be, at least partially, withdrawn into the body cavity, and this is effected by two retractor muscles which run from the posterior aspect of the septum to the body wall.

Some of the acanthocephalans (perforating acanthocephalans) can insert their proboscis in the intestine of the host and open the way to the abdominal cavity.<ref name="Parasites World" >{{cite web |title=Acanthocephalans drilling Acipenser stellatus intestine |publisher=Parasites World |url=http://parasites-world.com/acanthocephalns-drilling-acipenser-stellatus-intestine/ |access-date=August 3, 2009 |archive-date=April 30, 2012 |archive-url=https://web.archive.org/web/20120430062051/http://parasites-world.com/acanthocephalns-drilling-acipenser-stellatus-intestine/ |url-status=dead }}</ref>

=== Size ===

The size of these animals varies greatly, ranging from a few millimetres in length to ''[[Macracanthorhynchus hirudinaceus]]'', which measures from {{convert|10|to|65|cm|in}}. 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. [[Polyploid]]y is common, with up to 343n having been recorded in some species.

===Skin===

The body surface of the acanthocephala is peculiar. Externally, the skin has a thin [[tegument (helminth)|tegument]] covering  the [[Squamous epithelium|epidermis]], which consists of a [[syncytium]] with no [[cell wall]]s. The syncytium is traversed by a series of branching [[tubule]]s containing fluid and is controlled by a few wandering, [[amoeboid]] [[cell nucleus|nuclei]]. Inside the syncytium is an irregular layer of circular muscle fibres, and within this again some rather scattered longitudinal fibres; there is no [[endothelium]]. In their micro-structure the muscular fibres resemble those of [[nematode]]s.

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.

===Nervous system===
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 [[wiktionary:Papilla|papillae]] may possibly be sense-organs.