Acanthocephala: Characters, Sense Organs and Affinities

In this article we will discuss about Acanthocephala:- 1. General Characters of Acanthocephala 2. Habit, Habitat and External Characters of Acanthocephala 3. Body Wall 4. Proboscis Apparatus 5. Nervous System 6. Sense Organs 7. Excretory Organs 8. Ligament Sacs and Ligament Strand 9. Pseudocoel 10. Reproductive System 11. Copulation and Development 12. Affinities.

Contents:

1. General Characters of Acanthocephala
2. Habit, Habitat and External Characters of Acanthocephala
3. Body Wall of Acanthocephala
4. Proboscis Apparatus of Acanthocephala
5. Nervous System of Acanthocephala
6. Sense Organs of Acanthocephala
7. Excretory Organs of Acanthocephala
8. Ligament Sacs and Ligament Strand of Acanthocephala
9. Pseudocoel of Acanthocephala
10. Reproductive System of Acanthocephala
11. Copulation and Development of Acanthocephala
12. Affinitiesof Acanthocephala

1. General Characters of Acanthocephala:

The Acanthocephala are endoparasitic worms of slender cylindroid or slightly flattened form and hollow construction. They live as adults in the intestine of vertebrates and as larvae in arthropods. The diagnostic feature of the phylum is the organ of attachment consisting of an invaginable proboscis that forms the anterior end.

This proboscis is armed with rows of recurved hooks. The body wall consists of cuticle, syncytial epidermis permeated with spaces and sub-epidermal musculature. In connection with the proboscis apparatus the epidermis forms two elongated bodies termed lemnisci that hang down into the trunk.

Mouth, anus and digestive tube are completely wanting. There is no circulatory system. Excretory organs when present are of the nature of protonephridia and open into the terminal part of the reproductive system.

The nervous system consists of a ganglion near the proboscis and two lateral cords extend posteriorly from the ganglion along with numerous minor nerves. The sexes are separate, the females are generally larger than the males, the males are provided with a copulatory apparatus and the terminal part of the female apparatus is also somewhat complicated.

The eggs develop within the maternal body into a larva that requires an intermediate invertebrate host for its further development. There are over 500 known species. Echinorhynchus is the chief genus of Acanthocephala and it is described here in detail.

2. Habit, Habitat and External Characters of Acanthocephala:

The genus Echinorhynchus is a common parasite in the intestine of mammals, birds, reptiles, amphibians and fishes. The largest species E. (Gigantorhynchus) gigas is found in the pig and has once been recorded in the human being. It may attain the length of 50 to 65 cm.

External Characters of Acanthocephala:

The body is cylindrical and ends in front in a protrusible portion, the proboscis, which is cylindrical and is covered with many rows of recurved chitinous hooks.

In many species the body is ringed or constricted at regular intervals and so presents a more or less segmented appearance. Such segmentation reaches the highest degree in certain species of Moniliformis and Mediorhynchus.

In all these forms it is difficult to distinguish dorsal from ventral surface from external appearance when the body is curved, the concave surface is ventral and when the proboscis is covered with unequal size of hooks, the larger hooks are ventrally located, but in most cases dorsoventrality can be the proboscis includes the receptacle or sheath into which it invaginates or withdraws the lemnisci and the main ganglion of the nervous system.

The neck is generally short but sometimes much elongated.

The trunk may be cylindroid, flattened, curved or coiled with a smooth, wrinkled or segmented surface. In several genera the trunk is more or less noticeably differentiated into a broader fore trunk and a slender hind trunk in varying proportions in different species. There is no trace of mouth, anus or excretory pore. The gonopore occurs at or near the posterior extremity.

3. Body Wall of Phylum Acanthocephala:

The body wall is covered with stout cuticle of homogeneous structure. Beneath cuticle lies the remarkable epidermis or hypodermis. The epidermis is a thick layer of fibrous syncytial construction comprising three fibrous strata an outer layer, only slightly thicker than the cuticle, of parallel radial fibres, a middle somewhat thicker felt work of layers of fibres running in different directions and an inner layer of radial fibres.

The inner layer is the thickest of the three and by many authors is regarded as the epidermis proper, while the outer radial and the felt layers are assigned to the cuticle.

The fibres of the epidermis do not appear to be the nature of connective tissue but rather seem to be protoplasmic strands. There are no indications of cell walls and the entire epidermis forms a syncytium. The nuclei and the lacunar systems are situated in the inner radial layer.

In Acanthocephala the number of nuclei is approximately constant for each species, at least in early stages, and in many families throughout the life (Van Cleave, 1914).

These nuclei range in shape from globose or oval to rosette, amoeboid or highly branched forms and are of relatively large size up to 2 mm or more in length. The inner radial layer of epidermis contains the lacunar system, a set of channels without definite walls but having a more or less definite pattern.

The lacunar system consists of two longitudinal vessels with regularly spaced transverse connections. The main channels in this case are medially located, i.e., they are dorsal and ventral. The lacunar system is confined mainly to the epidermis and it does not communicate with the exterior or with any other body structure.

It contains a nutritive fluid presumably obtained by absorption Dissection of male, from the host and, therefore, serves as a food-distributing system in the absence of any other. The nutritive fluid in the system moves only with body movements.

The epidermis is followed by a thin layer, the dermis (also called basement or binding layer). It also permeates the underlying musculature. The muscle layer forming the innermost coat of the body wall is relatively thin consisting of outer circular and inner longitudinal fibres.

These muscle layers are also syncytial forming a fibrous network and probably contain a more or less definite number of nuclei. The muscle fibres consist of a cytoplasmic and a fibrillar portion; the fibres may run along one side of the cytoplasmic part or may encircle the latter. No definite lining membrane bounds the body wall from the pseudocoel.

4. Proboscis Apparatus of Acanthocephala:

The everted proboscis varies from a short cylindrical to globose shape to a long cylinder. It is armed with recurved hooks that are similar in shape but varied in size and arrangement (Van Cleave, 1941). They are most commonly arranged in alternating radial rows but may be concentric or irregular; they may be of even size over the proboscis.

The hooks may be largest at the summit and decrease gradually toward the base or often are largest in the middle region decreasing towards the end. The number of rows, the number per row, and the size pattern of the hooks are fairly constant for each species, and in fact the proboscis armature constitutes one of the most important taxonomic characters of the phylum.

The larger hooks have roots sunk into the proboscis wall and some authors reserve the term hooks for these, calling the smaller, rootless members of the armature spines. The hooks and spines are of unknown chemical nature but apparently consist of the same material as the dermis from which they seem to originate. They are covered with cuticle.

The proboscis is invaginable and withdraw-able into a muscular sac, the proboscis receptacle that is fastened in a circle to the inner surface of the proboscis wall and hangs into the pseudocoel.

It is composed in some groups of a single muscle layer, in others of two layers. Proboscis and receptacle are operated by special muscle bands. From the proboscis summit there extends posteriorly along the proboscis interior a muscle or group of muscles termed the retractor, invaginator or invertor muscle.

This inserts on the receptacle wall and also passes through this wall to continue as the dorsal and ventral receptacle retractors that terminate on the trunk wall. In the more complicated types there are also a number of dorsal, ventral and lateral receptacle protrusors that originate in a circle from the neck wall and insert on the rear part of the receptacle.

Finally generally distributed in the Acanthocephala there are the retractors of the neck that originate near the posterior boundary of the neck and insert on the trunk. The retractors of the neck encircle all the other proboscis muscles and embrace the lemnisci as compressors of lemnisci.

From the posterior part of the neck region there extend posteriorly into the trunk pseudocoel a pair of projections of the inner radial layer of the epidermis known as lemnisci. The lemnisci are generally long slender bodies, they are supplied by vessels of the lacunar system and contain either a limited and definite number of fragmented nuclei.

They are externally covered by the dermis and are well supplied with the lacunar system. At certain levels the lemnisci are enclosed in the neck retractors. The function of the lemnisci is that they act as reservoirs for the fluid of the lacunar system of the presoma when the proboscis is invaginated.

5. Nervous System of Acanthocephala:

The nervous system consists of the cerebral ganglion, the branches from this and in the male a pair of genital ganglia with branches. The cerebral ganglion, often called simply the ganglion, is a large cellular mass enclosed in the proboscis receptacle in contact with its ventral walls. It consists of a central fibrous mass embraced by ganglion cells, 86 in Macracanthorhynchus, 80 in Hamanniella and 73 in Bolbosoma.

In the first two genera, the ganglion gives off two single and three pairs of nerves, in Bolbosoma one single and five pairs of nerves.

The former comprise an anterior median and a ventral anterior nerve to the musculature and sensory papillae of the proboscis, a pair of lateral anterior nerves to the lateral protrusors, a pair of lateral medial nerves to the receptacle wall, and the pair of main lateral posterior nerves that proceed to the posterior end of the animal.

The lateral posterior nerves pierce the receptacle wall, proceed laterally to the body wall of the trunk into which they give branches, and then run posteriorly in the lateral body wall in the longitudinal muscle layer to the posterior end, giving off genital branches in females.

In males, however, branches from these nerves enter a pair of genital ganglia situated in the penis base and connected with each other by a ring commissure. From the genital ganglia branches proceed anteriorly along the male genital tract and posteriorly into the bursa where some terminate in bulbous sense organs.

6. Sense Organs of Acanthocephala:

Organs of special sense are not developed in correlation to entoparasitic life.

The known sense organs comprise three in the proboscis and several in the male bursa and penis. In the proboscis there is a sensory organ in the centre of the tip and in some genera one on each side in the neck. The terminal sense organ consists of a small pit beneath which there is a fusiform nerve ending of a nerve fibre that makes a coil just below its termination.

The pair of lateral proboscis sense organ is similar except that several coiled nerve fibres are involved.

These proboscis sense organs are supplied by certain of the anterior nerves coming from the cerebral ganglion. In males fibres from the genital ganglia terminate in bulbous or spherical sense organs of which there are seven or eight around the rim of the bursa and a number in the penis. It is usually supposed that all the acanthocephalan sense organs are of tactile nature.

7. Excretory Organs of Acanthocephala:

The excretory organs consist of a pair of small bodies, protonephridia, situated at the posterior end near the genital aperture. In most genera each protonephridium consists of a branching mass of flame bulbs attached to a common stem.

The number of flame bulbs in each protonephridium ranges from about 250 to 700. The flame bulbs are devoid of nuclei and are, therefore, not cells; usually three nuclei occur in the main branches or in the wall of the chamber.

The flame bulb consists of a linear row of cilia. In certain genera (Oligacanthorhynchus, Nephridiorhynchus), the flame bulbs open directly into a sac from which the nephridial canal leads (Meyer, 1931).

In any case the two canals unite to a single canal or to a bladder and this joins the common sperm duct in the male and uterus in the female. The terminal canals of the reproductive system are then urinogenital canals in the Archiacanthocephala.

8. Ligament Sacs and Ligament Strand of Acanthocephala:

These are structures peculiar to the Acanthocephala. The ligament sacs (formerly called ligaments) are hollow tubes of connective tissue with or without accompanying muscle fibres that run the length of the body interior and enclose the reproductive organs.

Anteriorly they are attached to the posterior end of the proboscis receptacle or the adjacent body wall and posteriorly they terminate on or in some part of the reproductive system. In the female there are two ligament sacs, a dorsal and a ventral, whose medial walls make contact in the frontal plane and which communicate anteriorly by an opening.

The dorsal ligament sac attaches posteriorly to the anterior rim of the uterine bell, the ventral sac extends to the posterior end of the body, opening en route into the ventral aperture of bell. In males there is only one sac, the ventral sac is wanting. The dorsal sac encloses the testes and the cement glands and posteriorly becomes continuous with the genital sheath.

Haffner (1942) has reported the ligament strand, a nucleated strand found between the two ligament sacs when present or situated along the ventral face of the single ligament sac. The gonads in both sexes are attached to this strand. According to Haffner’s analysis, the ligament strand represents the endoderm mid-gut. The ligament sacs are regarded by Hyman as separated parts of the pseudocoel.

9. Pseudocoel of Acanthocephala:

The pseudocoel is a cavity, not provided with any lining membrane, between the body wall and the ligaments. It is small in forms with two ligament sacs but attains considerable size in those with one sac only. It also extends into the presoma between the muscle bands.

As the cavity lacks a lining membrane, it is obviously not a coelom. As the ligament strand apparently represents the endoderm, the body cavity is a space between the endoderm and the body wall and, hence, classified as a pseudocoel. The pseudocoel is filled with a clear fluid.

10. Reproductive System of Acanthocephala:

The greater part of the body is occupied by the reproductive organs. The sexes are separate and the female is larger than the male. In both the sexes the gonads and their ducts are connected with a ligament strand which extends backwards from the end of the proboscis sheath.

In males there are two oval, rounded or elongated testes enclosed in the ligament sac and attached to the ligament strand. From each testis a sperm duct proceeds posteriorly inside the ligament sac. Small enlargements representing spermiducal vesicles may occur along the sperm ducts.

A cluster of unicellular gland cells known as cement glands (usually six or eight in number and of variable shape-rounded, pyriform, clavate or tubular) open into the sperm duct shortly behind the more posterior testis. The ducts of these cement glands, either separately or after union into one or two main ducts, enter the common sperm duct.

The sperm ducts, the cement ducts and the protonephridial canals (when present) are all enclosed in a muscular tube, the genital sheath.

The genital sheath terminates on the muscle cap of bursa. Inside the genital sheath, the two sperm ducts unite to a common sperm duct which may present a saccular enlargement, the seminal vesicle, the cement duct enter the common sperm duct and the common protonephridial canal, when present, also unites with common sperm duct.

The urogenital canal so formed penetrates the centre of the penis, a short conical protrusion.

The penis projects into hemispherical or elongated cavity the bursa, that is eversible to the exterior and grasps the rear end of the female in copulation. The bursa is composed of internal body wall of which the muscular layer is greatly thickened in the proximal part of the bursa, forming the muscular cap. The sperms are long filaments without definite heads.

The female reproductive system departs from the usual in many ways. The original single or double ovary breaks up into fragments termed ovarian walls that float free in the dorsal ligament sac but as the latter sac soon ruptures the balls occupy the pseudocoel. The ligament sacs lead to the first part of the female canal termed the uterine bell.

The uterine bell is a muscular, funnel- shaped or tubular organ that by peristaltic contractions engulfs the developing eggs and passes them onward.

The bell has a single (or a pair of) posterior ventral openings through which the immature eggs, which are spherical, pass back into the body cavity. At its posterior end the bell narrows to a uterine tube composed of several large cells with conspicuous nuclei and bearing two bell pouches that extend anteriorly.

The uterine tube enters the uterus a muscular tube of some length and this is followed by the short non-muscular vagina opening to the exterior.

The nephridia lie alongside the uterine bell, the two protonephridial ducts run in the dorsal wall of the bell and the common canal formed by their union opens into the beginning of the uterine tube. The ovarian balls consist of central syncytium from which ovogonia separate, passing to the periphery for further development.

11. Copulation and Development of Acanthocephala:

In copulation the everted male bursa grasps the posterior end of the female and the penis enters the vagina and discharges sperms into the uterus. This is followed by the discharge of the cement secretion which sets as a plug in the gonopore and as a cap over the whole posterior tip, preventing escape of the sperms.

The mature ova are elliptical and surrounded by a membrane. Fertilisation takes place in the body cavity and after fertilisation a membrane arises inside the original egg membrane.

In the meantime the eggs have escaped from the ovarian balls and continue development in the pseudocoel or inside the dorsal ligament sac until a larval stage provided with a rostellum armed with hooks is reached. Meanwhile, a third membrane, usually termed shell, has formed between the two membranes already present around the embryo.

These ovic larvae are engulfed by the uterine bell and passed towards the uterine tube; those not sufficiently mature may be returned through the ventral bell aperture into pseudocoel or ventral ligament sac. The ripe ones proceed into the bell pockets and then along the uterus and vagina to the exterior. These elliptical ovic larvae must be ingested by the proper invertebrate host before they can develop further.

12. Affinities of Phylum Acanthocephala:

Because of many peculiarities of structure of Acanthocephala and the parasitic degeneration or alternation of some of their systems, determination of their affinities poses a difficult problem. The acanthocephalan worms were noticed about the beginning of the 18th century but were clearly distinguished from other intestinal worms until 1771 when Koelreuther proposed the name Acanthocephalus for one from fish.

In 1776, Zoega and O.F. Muller without knowledge of Koelreuther’s work gave the name Echinorhynchus to a similar fish parasite. Zeder (1803) gave these worms a common name “Haken wurmer” (hooked worms) and Rudolphi (1809) changed this into the form Acanthocephala (Greek, akantho = spiny; kephalo = pertaining to the head) by which the group has since been known.

Cuvier included the acanthocephalan worms with the flatworms in his group of parasitic worms. Vogt (1851) first clearly distinguished the flatworms from the roundworms upon which Gegenbaur gave the name Nemathelminthes to the roundworms. The position of Acanthocephala, however, has remained uncertain.

At the present time there are only two views that need be seriously considered, those associating the Acanthocephala with the Platyhelminthes or with the Aschelminthes. The best way to decide between these two possibilities is to discuss, system by system, the similarities and dissimilarities between the three groups, something that has also been done by Chitwood (1940) and Van Cleave (1941).

Similarities with Platyhelminthes:

The Acanthocephala resemble with the Platyhelminthes in the following respects:

(1) An armed and protrusible proboscis occurs in certain cestodes and the proboscides of Trypanorhyncha and Acanthocephala is similar so far as the shape and arrangement of hooks is concerned.

(2) Presence of cuticle and syncytial nucleated epidermis.

(3) Musculature with circular as well as longitudinal fibres.

(4) Protonephridia of flame-bulbs are present.

(5) Reproductive system, particularly in male, resembles that of many flatworms.

(6) Embryology is like that of cestodes.

(7) Serological tests indicate a relationship of Acanthocephala with cestodes rather than nematodes.

Similarities with Aschelminthes:

The following similarities are noteworthy:

(1) The division of the body into the presoma and the trunk as in priapulids and the gordiacean larva.

(2) An armed proboscis is found among the Aschelminthes in echinoderids, priapulids and gordiacean larva.

(3) Superficial segmentation, sometimes involving musculature, is also conspicuous in rotifers, echinoderids, priapulids and nematodes, etc.

(4) Clothing of body with cuticle and syncytial nucleated epidermis.

(5) Division of pseudocoel by partitions and tissues resembling mesenteries.

(6) Reduction of gut to a strand is found in male rotifers.

(7) Flame-bulb protonephridia present.

(8) A close relationship of nephridial and reproductive systems is common in priapulids and rotifers.

Differences from Nematoda:

Acanthocephala, however, differs from the Nematoda in the following points:

(1) Presence of proboscis.

(2) Absence of digestive tract.

(3) Presence of circular muscles.

(4) Presence of ciliated excretory organs.

(5) Peculiarities and complexities of reproductive system.

The foregoing comparisons do not furnish a decisive answer to the question whether the Acanthocephala are allied to the Platyhelminthes or to the Aschelminthes. The general structure is rather on the aschelminthic side, whereas the embryology presents more points of resemblance with the Platyhelminthes.

Chitwood (1940) and Van Cleave (1941) favour a platyhelminth affinity, and on this basis Chitwood (1940) has proposed a superphylum Parenchymata to embrace the flatworms, nemertines and acanthocephalans.

Leaving apart the fact that the Acanthocephala are not parenchymatous animals, one doubts the utility of the superphylum concept. On present evidence, raising the Acanthocephala to the rank of an independent phylum appears the best disposition of the group.