Phylum Acanthocephala: Characters, Classification and Affinities

In this article we will discuss about Phylum Acanthocephala:- 1. Characters of Phylum Acanthocephala 2. Classification of Phylum Acanthocephala 3. Affinities.

Characters of Phylum Acanthocephala:

Phylum Acanthocephala includes a large number of parasitic worms. The most typical forms are Acanthocephalus, Neoechinorhynchus, Gigantorhynchus. They are parasites in the intestine of different vertebrates ranging front fish to mammals.

The anterior end of the cylindrical body is produced into an exten­sible proboscis beset with rows of numerous chitinous recurved hooks (Fig. 15.21). Two club-shaped fluid-filled structures called lemnisci are hanging at the sides of the probos­cis. The lemnisci act as hydraulic system in proboscis eversion.

A distinct neck between the proboscis and the trunk is present in certain forms. There is no aperture in the body excepting the gonopore at the poste­rior end of the body. The alimentary canal is totally absent. Two longitudinal vessels are present in the body wall which probably help in absorption and distribution of nutri­tion.

A single large nerve ganglion is situ­ated at the base of the proboscis. Sense organs are lacking. The reproductive organs occupy almost whole of the body cavity. A pair of excretory organs is present in the posterior side of the body cavity. Single median excretory duct opens into the oviduct in the female and into the ejaculatory duct in the male (Fig. 15.21B).

The sexes are separate. There are two oval testes which remain connected with a sus­pensory ligament sac. It is a hollow, elon­gated sac of connective tissue, hangs in the pseudocoel from the proboscis sac to the accessory reproductive organs.

Similarly, ovary is also connected with the suspensory ligament. In mature females, the ovary frag­ments into numerous bodies called ovarian balls that float freely in the ligament sac fluid.

The early advanced phase of develop­ment takes place in the pseudocoel. The embryo is encased within a sac and the anterior end is provided with hooks. This stage is called acanthor stage. The acanthor larva contains a hooked rostellum that is used in penetrating host’s tissue. At this stage the embryo is extruded from the intes­tine of the host along with the faeces.

Further development is only possible if the embryo is swallowed by an intermediate host. The intermediate host in all the forms are some Arthropods.

After reaching into the intestine of the intermediate host, the chitinous mem­branes dissolve and the embryo undergoes further development. The embryo either re­mains fixed to the intestinal wall and mi­grates into the body cavity. If the intermedi­ate host is taken by the permanent host, attainment of adult size and sexual maturity are achieved.

Classification of Phylum Acanthocephala:

Phylum Acanthocephala:

[Acanthocephala; Gk. acantha = thorn + kephale = head, or called thorny-headed worms]. Approx. more than 1200 species.

Characteristics of the Phylum:

1. Un-segmented, worm-like, parasitic aschelminthes.

2. Length of the body less than 10 mm in most cases.

3. Body divisible into a trunk and a short, hollow, fluid-filled, retractile proboscis with recurved hooks.

4. Body surface often with numerous, smaller spines.

5. No cuticle.

6. Thick, syncytial nucleated tegument (non-ciliated outer syncytial layer of the body wall) is present.

7. Circular and longitudinal muscle fi­bres present beneath the epidermis.

8. Presence of a large pseudocoel di­vided by partitions and tissues.

9. Digestive tract absent.

10. A unique circulatory system, the fluid- filled lacunar canal system is present in the epidermis.

11. Nervous system consists of an ante­rior brain with lateral nerves.

12. Excretory system is paired protonephridia consisting of bundles of flame cells in some species.

13. Sexes separate (= gonochoristic).

14. Saccular gonads with thin ligament sacs.

15. Males with cement glands whose se­cretion help to plug the female’s vagina.

16. Usually two testes remain one behind the other on the ligament in males.

17. A cup-shaped copulatory bursa at the posterior end of the males with a protrusible penis.

18. Fertilization internal.

19. Spiral cleavage (a modified form).

20. Vertebrate parasites.

21. Life cycle completed through the two hosts; larval stage completed through arthropods and adults within the in­testine of vertebrates.

The phylum Acanthocephala is divided into 3 classes:

1. Archiacanthocephala,

2. Eocanthocephala and

3. Palaeacanthocephala.

Class 1. Archiacanthocephala:

1. Parasites of birds and mammals; in­sects, centipedes and millipedes act as intermediate hosts.

2. Females about 70 cm in length and are 3 to 4 times larger than the males.

3. The spines of the proboscis are con­centrically arranged.

4. Main lacunar canals are median.

5. Protonephridia are present.

Examples:

Gigantorhynchus, Macracanthorhynchus.

Class 2. Eoacanthocephala:

1. Most species parasitize fish, some amphibians and reptiles. Crustaceans serve as intermediate hosts.

2. The spines on proboscis are radially arranged.

3. Lacurinar canals are median.

4. Protonephridia are absent.

Examples:

Neoechinorhynchus, Octospimfer.

Class 3. Palaeacanthocephala:

1. Definite hosts include fish, amphi­bians, reptiles, birds and mammals. Crustaceans usually serve as interme­diate hosts.

2. The spines are alternately arranged on proboscis.

3. Lacunnar canals are lateral.

4. Protonephridia are lacking.

Examples:

Acanthocephalus, Polymorphus.

Affinities of Phylum Acanthocephala:

The parasitic mode of life and structural peculiarities pose a problem in the determi­nation of affinities of Phylum Acanthocephala. Koelreuther recorded the acanthocephalan worms from the intestine of fish as a parasite and proposed the name Acanthocephalus in 1771.

Cuvier placed the acanthocephalans with the flatworms. Vogt distinguished the flatworms from the roundworms in 1851, and Gegenbaur gave the name Nemathelminthes for the roundworms. At present two groups of zoologists are inclined to place the Acanthocephala either with Platyhelminthes or with Aschelminthes.

Affinities of Phylum Acanthocephala with Platyhelminthes:

Similarities:

1. Armed contractile proboscis occurs in certain cestoideans.

2. Presence of syncytial tegument in Cestoideans, Digenea and Monogenea.

3. Circular and longitudinal muscle fi­bres present.

4. Absence of digestive tract.

5. Protonephridia with flame cells are present.

6. Male reproductive system is compara­ble with that of many flatworms.

7. The hooked rostellum of the larva is comparable with the hexaconth km of the Cestoideans.

Affinities of Phylum Acanthocephala with Aschelminthes:

Similarities:

1. The division of the body into probos­cis and trunk is comparable with the gordioid larva of Nematomorpha.

2. An armed retractile proboscis occurs in kinorhynchs and gordiacean larva of Nematomorpha among Aschelminthes.

3. Syncytial tegument present.

4. Presence of circular and longitudinal muscle fibres.

5. Presence of pseudocoel, divided by partitions and tissues resembling mesenteries.

6. Presence of protonephridia with flame cells.

7. Eutelic condition.

Affinities of Phylum Acanthocephala with Nematoda:

Acanthocephalans are considered as a member of non-coelomate groups which are included nematoda, Nematomorpha, Acan­thocephala, Rotifera and Calyssozoa (Endoprocta).

Though Acanthocephalans show some similarities with Nematoda but they differ in following dissimilarities:

Dissimilarities:

1. Presence of recurved spined proboscis.

2. Absence of digestive tract.

3. Presence of circular muscle fibres.

4. Presence of protonephridia with cili­ated flame cells.

5. Complex reproductive system.

Affinities of Phylum Acanthocephala with Rotifera:

Similarities:

1. Both phyla lack a cuticle.

2. Protonephridia with bundles of flame cells are present in some acantho­cephalans, compare with rotifers.

3. Epidermis syncytial in acanthocephalans shows the similar intracel­lular network of protein fibres of rotifers.

4. Structure of the sperm.

Remark:

Comparisons of 18S ribosomal gene sequences support a closely evolutionary relation­ship between the acantho­cephalans and rotifers and the two groups should be placed under a single phylum and may be called Syndermata.