PHYLUM ACANTHOCEPHALA (akantha, spine + kephale, head)
1.Habitat: Adult acanthocephalans are endoparasite in the intestinal tract of vertebrates (especially fishes). They complete their life cycle in two hosts. The juveniles are parasites of crustaceans and insects. Zoologists have identifiedabout a thousand species.
2. Size: Acanthocephalans are generally small. They are less than 40 mm long). However, one species Mucracamhorhynchus hirudinaccus can be up to 90 cm long. It occurs in pigs.
3. Body form: The body of the adult is elongate. Body is composed of a short anterior proboscis, a neck region and a trunk. Trunk is covered with recurved spines. Therefore, they are named as spiny-headed worms. The retractable proboscis is used for attachment in the host’s intestine. Females are always larger than males.
4. Body wall: Body wall is covered by a living syncytial tegument. It is an adaptation to the parasitic way of life. A glycocalyx covers the tegument. It consists of mucopolysaccharides and glycoproteins and protect against host enzymes and immune defenses.
5.Digestive system: Digestive system is absent in acanthocephalans. They absorb food directly through the tegument from the host. Protonephridia may be present
6. Nervous system: The nervous system is composed of a ventral, anterior ganglionic mass. Anterior and posterior nerves arise form it. Sensory organs are poorly developed.
7. Reproduction: The sexes are separate. The male has a protrusible penis. Fertilization is internal. Eggs develop in the pseudocoelom.
8. Development: The eggs pass out of the host with the feces. These eggs must be eaten by certain insects like cockroaches or grubs (beetle larvae). They are also eaten by aquatic crustacean (e.g. amphipods. isopods, ostracocis). The larva emerges from the egg in the host. It is now called an acanthor. It burrows through the gut wall and reaches the hemocoel. It develops into an acanthella in hemocoel. It finally changes into a cystacanth. Mammal, fish or bird eats the intermediate host. The cystacanth comes out of cyst. It attaches to the intestinal wall with its spiny proboscis and develops into adult.