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An Introduction to Zoology: Chapter 12
An Introduction to Zoology: Chapter 12
Published by The Lone Ranger
01-05-2009
Default Phylum Acanthocephala


Phylum Acanthocephala:
Members of the phylum Acanthocephala (from the Greek “akantha,” meaning “spine” or “thorn,” and “kephale,” meaning “head”) are commonly known as “spiny-headed worms.” There are about 1,100 species known. You probably wouldn’t guess it from looking at them, but studies have indicated that acanthocephalans are so genetically-similar to rotifers that perhaps they should properly be classified as highly modified members of the phylum Rotifera. (Some authors call acanthocephalans “evil rotifers.”)


Most acanthocephalans are only a few millimeters long, but a few species can be more than a meter long at maturity. All known acanthocephalans are endoparasites. Adults live in the intestines of vertebrates, especially fishes. Larval acanthocephalans parasitize arthropods – either insects or crustaceans, depending on the species.

The most distinctive feature of an adult acanthocephalan is the cylindrical proboscis that projects from the worm’s anterior end. The proboscis is invaginable – that is, it can be inverted and withdrawn into the body cavity – but its most noticeable characteristic is that it’s covered in recurved spines. These spines allow adult acanthocephalans to attach themselves to the intestinal linings of their hosts.


Like its rotifer cousins, an acanthocephalan has a syncytial epidermis, properly known as the tegument. Also like rotifers, acanthocephalans have no respiratory or true circulatory systems. Gas exchange occurs across the tegument. Smaller acanthocephalans lack any sort of excretory system, but larger species possess protonephridia with flame cells, just as do many rotifers.

While there is no heart and no true circulatory system, a series of fluid-filled canals called a lacunar system extends through the tegument and even into body muscles. Contraction of body muscles helps to move fluid through these canals, and the fluid may serve to transport nutrients, gases and metabolic wastes throughout the body in somewhat the same way that a true circulatory system does.

An acanthocephalan has no digestive tract. Since it lives inside the digestive tract of its host, it can absorb the nutrients it requires across its tegument, in much the same way that a cestode flatworm does.

A spiny-headed worm has an extremely simple nervous system and very few sense organs. You would probably expect this, given that the worm spends its entire life inside the bodies of other animals – it isn’t as if it has to hunt for food or worry about evading predators.


Like many other endoparasites, some acanthocephalans can apparently manipulate their hosts’ behaviors. In particular, many acanthocephalan species appear to manipulate the behavior of their intermediate hosts in such a way as to increase their chances of getting into their definitive hosts.

Consider Gammarus lacustris, a small aquatic crustacean found in freshwater lakes and streams. These animals are preyed upon by ducks, so you probably wouldn’t be surprised to know that they normally avoid light. During daylight hours, they will burrow into the lake bottom, hide under pebbles, or otherwise avoid light. In this way, the crustacean reduces its chances of being seen and caught by a duck.

As it happens, Gammarus lacustris is the intermediate host for acanthocephalans that have ducks as their definitive hosts. So, any acanthocephalan that’s parasitizing one of these crustaceans “wants” its host to be eaten by a duck. How to accomplish this?


Well, during their mating season, Gammarus lacustris individuals produce large amounts of the neurotransmitter serotonin. Among other things, high serotonin levels seem to suppress Gammarus lacustris’ normal photophobic (“light-fearing”) behavior. Acanthocephalans parasitizing these crustaceans appear to manipulate their hosts’ serotonin levels. The result is that these normally-photophobic animals are actually attracted to light. Since infected crustaceans will swim or climb toward light, they’re easily seen by – and caught by – ducks.

And thus, the worm gets from its intermediate host to its definitive host.



You’ll be glad to learn that there are no species of acanthocephalans that normally parasitize humans. Some species have pigs or other large mammals as their definitive hosts, however, and can occasionally parasitize humans. Several acanthocephalan species have soil-dwelling beetle larvae as their intermediate hosts, and children who eat dirt sometimes become parasitized by acanthocephalans. All in all, it’s fortunate that they rarely parasitize humans, because spiny-headed worms can do serious damage to their hosts’ intestines.



An acanthocephalan with its proboscis everted. As you would
expect from an animal that lives inside the intestine of another
animal, an acanthocephalan’s internal anatomy is very simple.




Close-up view of a spiny-headed worm’s proboscis. You can
see why you wouldn’t want any of these critters in your intestine.




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Thanks, from:
Ensign Steve (01-05-2009), monruw (03-30-2011)
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