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In this article we will discuss about Entoprocta:- 1. General Characters of Entoprocta 2. Structure of Entoprocta 3. Body Wall 4. Pseudocoel 5. Digestive System 6. Excretory System 7. Nervous System 8. Sense Organs 9. Reproductive System 10. Affinities.
Contents:
- General Characters of Entoprocta
- Structure of of Entoprocta
- Body Wall of Entoprocta
- Pseudocoel of Entoprocta
- Digestive System of Entoprocta
- Excretory System of Entoprocta
- Nervous System of Entoprocta
- Sense Organs of Entoprocta
- Reproductive System of Entoprocta
- Affinities of Entoprocta
1. General Characters of Entoprocta:
The Entoprocta are sessile organisms and their body is divided into a rounded or oval mass known as calyx. Calyx contains all the viscera. The slender stalk, that bears the calyx, is attached basally to some object or to another animal. On the free edge of the calyx is borne a circlet of tentacles ciliated on their inner faces.
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The surface of the calyx bordered by the tentacles is regarded as ventral and, hence, the convex surface from which stalk extends is dorsal. Mouth and anus open on the free surface of the calyx inside the tentacles at opposite ends of sagittal axis and the digestive tract is strongly curved into a U-shape.
The central part of the nervous system consists of a ganglionic mass located in the concavity of the digestive tract. There is a pair of protonephridia, each provided with a single flame bulb. The majority of the Entoprocta are dioecious but a few species are hermaphroditic. The two gonads lie in the pseudocoel to either side of the terminal part of the intestine and open by a common gonopore shortly anterior to the anus.
The yolky egg develops in a brood chamber formed of the calyx surface around the gonopore into a trochophore type of larva that after being set free attaches and undergoes a process of metamorphosis into the adult condition. Extensive reproduction by asexual processes is characteristic of the phylum.
The Entoprocta are solitary or colonial, free-living or epizoic. They are marine with the exception of the genus Urnatella found in freshwater and are widely distributed. There are about 60 known species.
2. Structure of Entoprocta:
The Entoprocta are small, almost microscopic animals below 5 mm in length. They either grow singly or in colonies and are found attached to objects or to other animals and having the general appearance of hydroid polyps. The main features of the entoproct are the crown of tentacles, the body mass or calyx, the stalk and the basal attachments of the stalk.
The calyx is somewhat flattened laterally, the tentacular crown is oval or elliptical in outline. The number of tentacles ranges from 8 to 30 in different species. The tentacles are usually of the same length throughout the crown but in some loxosomatids, there are four longer tentacles at the oral end of the crown.
The tentacles are evenly spaced except that there is a wider gap at the oral and anal ends and this confers a bilateral symmetry upon the tentacular crown. The tentacles are ciliated on their inner surfaces and this ciliation is related to a ciliated vestibular groove that runs along the inner side of the tentacular bases. Outwardly the tentacular bases are connected by a tentacular membrane that forms the edge of the calyx.
The calyx or body mass is slightly compressed laterally and is oriented either at right angles to the stalk obliquely to it. Its free flattened or concave surface is ventral and the convex attached surface is dorsal. Mouth and anus open on the ventral surface at opposite ends of the sagittal axis, inside the tentacles.
When the calyx is placed obliquely on the stalk, the downward tilt is towards the mouth and the anus may attain still greater elevation by being mounted on an anal cone. Posterior to the mouth in the sagittal axis is located the nephridiopore and posterior to that the gonopore.
The concavity between the mouth and anus is called vestibule (also atrium) and the part of this that lies between gonopore and the anus serves as brood chamber for the developing eggs. The outer or dorsal surface of the calyx is usually smooth.
The stalk is an outgrowth or elongation of the calyx. It offers much variation throughout the group. The stalk may be smooth or spiny. In several genera the stalk base shows a pronounced muscular enlargement and similar enlargements may occur along the stalk and near the calyx.
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These muscular swellings, acting like sockets, permit the odd flicking, bowing movements characteristic of such entoprocts and are also related to asexual reproduction.
3. Body Wall of Phylum Entoprocta:
The structure of the body wall is typically pseudo coelomate type and is practically the same throughout stalk, stolon’s, calyx and tentacles. The body surface is clothed with a cuticle except on the tentacles and vestibule. Beneath the cuticle lies the cellular epidermis consisting of a single layer of mostly cuboidal cells.
The epidermis is taller on the inner surface of the tentacles and along the vestibular groove in which locations it is also heavily ciliated. Numerous large gland cells occur in the epidermis of the calyx in loxosomatids. They are of two types a granular opaque type and a transparent type filled with vacuoles. To the inner side of the epidermis occurs the body wall musculature in the form of longitudinal fibres.
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This musculature is sparsely present in the calyx. Muscle strands are present along the inner wall of the tentacles and enable these to be curved inward towards the vestibule. A band of muscle fibres causes within the tentacular membrane forming a sphincter that contracts this membrane over the tentacular crown when the latter is curled into the vestibule.
4. Pseudocoel of Entoprocta:
The pseudocoel occupies the interior of tentacles, stalks, stolon’s and the space in the calyx between body wall and digestive tract. It is everywhere filled with a gelatinous material containing mesenchyme cells. The pseudocoel of tentacles is filled with large rounded cells, free amoeboid cells are also present.
The pseudocoel of the calyx is separated from that of the stalk not only by the septum but also by plug of cells blocking the central hole in the septum. The pseudocoel of the stalk and stolon’s contains some wandering amoeboid cells but is mostly filled long, fusiform, more or less rigid cells known as tube cells.
5. Digestive System of Phylum Entoprocta:
The U-shaped digestive tract occupies the greater part of the interior of the calyx. It is bilaterally symmetrical with respect to the sagittal plane of the calyx. The mouth is situated at the anterior end of the ventral surface of the calyx. It is a transversely elongated ciliated aperture located just within the tentacles.
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The mouth leads into a funnel-shaped buccal cavity that narrows into the tubular oesophagus. Oesophagus opens into the enlarged sacciform stomach, the most conspicuous organ of the entoproct, occupying most or part of the curve of the U. From the stomach the narrow intestine proceeds ventrally and is usually separated by a constriction from the terminal rectum.
The rectum opens by the anus at the posterior end of vestibular surface, well inside the tentacles. The anus is often mounted on a projecting eminence, the anal cone. The digestive tract consists throughout of a one-layered ciliated epithelium of varying height. The digestive tract lacks a muscular coat but there is a limited amount of musculature associated with it.
The fibres originating on the calyx wall and inserted on the lower lip act to expand the mouth opening. The oesophagus can be constricted by fibres that encircle it and extend from its posterior surface to the calyx wall and expanded by other fibres from this surface to the lateral calyx wall. Sphincter fibres occur at the junction of intestine and rectum and around the anus.
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6. Excretory System of Phylum Entoprocta:
The Entoprocta are provided with a single pair of flame bulbs in each calyx, lying ventral to the stomach between the oesophagus and the sub-enteric ganglion.
The intra-cellular canal from each bulb forms a channel through a few enlarged cells often provided with amoeboid extensions and apparently functioning as athrocytes. The two ducts then converge and unite before opening by a single nephridiopore situated in the median line shortly posterior to the mouth.
7. Nervous System of Phylum Entoprocta:
The central nervous system consists of one main ganglionic mass located ventral to the stomach, between the stomach and the vestibular wall. It apparently represents a sub-enteric ganglion. The ganglion is of rectangular to bilobular shape and consists peripherally of ganglion cells, centrally of fibres. From the ventral surface of the ganglion three pairs of nerves proceed to supply the crown of tentacles.
Their branches each terminate in a large ganglion cell or group of cells situated in the tentacular membrane between the tentacle bases. From each such ganglion nerves are given off into adjacent tentacle. From the dorsal surface of the sub-enteric ganglion spring a pair of nerves to the calyx wall, a pair to the stalk and a small pair to the adjacent gonads.
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8. Sense Organs of Phylum Entoprocta:
Sensory nerve cells of tactile type, consisting of one or more bristles proceeding from a nerve cell situated beneath the epidermis and piercing the latter are abundant on the outer surface of the tentacles and along the calyx margin.
In loxosomatids either throughout life or in larval stages only, there occurs on the sides of the calyx near its oral end a pair of sense organs bearing a remarkable resemblance to the antennae of rotifers. Each consists of a tuft of bristles lined by a ganglion, from which a nerve proceeds to a ganglionic mass connected in turn with the sub-enteric ganglion.
9. Reproductive System of Phylum Entoprocta:
Some entoprocts are hermaphroditic and others are dioecious. But it is suspected that at least some apparently dioecious species may be actually protandric hermaphrodites. The gonads are a single pair of sacciform bodies located ventral to the liver region of stomach, either anterior or posterior to the ganglion. In hermaphroditic species, there is a pair of testes posterior to the pair of ovaries.
From each gonad a short duct proceeds medially and unites with its fellow to open on the ventral surface of the calyx by a common gonopore.
In hermaphrodites the sperm duct unites with the oviduct of that side prior to the formation of the common duct. The gonoducts are beset with eosinophilous unicellular glands or have an area of glandular epithelium. In males the common sperm duct may present an enlargement, the seminal vesicle, for the storage of ripe sperms.
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The sperms are flagellate type. The common gonopore is medially located behind the nephridiopore at the base of a pronounced elevation so that calyx surface between this elevation and the anal cone forms a considerable depression, the genital recess, which in females and hermaphrodites, acts as a brood chamber for the developing eggs.
(i) Asexual Reproduction:
All entoprocts proliferate extensively by asexual budding. In the Loxosomatidae buds arise on the sides of the calyx near its oral end in a pair of bilaterally symmetrical areas. In the Pedicellinidae buds are produced only by the stolons and stalks, never by the calyces and as the buds remain attached colony formation results.
The bud begins as an epidermal proliferation which cuts off into the interior as an epithelial vesicle. This soon constricts into two vesicles of which the outer one becomes the free surface of the calyx and the tentacular crown and proliferates the ganglion from its inner wall and the inner vesicle develops into the digestive tract.
The muscles, gonads and other mesodermal structures arise from the parental mesenchyme included in the bud. A constriction then separates the outgrowth into calyx and stalk. In the asexual reproduction the entire organism originates from the ectoderm and mesoderm without any participation of the endoderm.
(ii) Regeneration:
The entroprocts possess good powers of regeneration. Under adverse conditions colonies shed their calyces but the stalks and stolons remain alive for considerable periods and regenerate new calyces upon return of favourable conditions.
(iii) Sexual Reproduction and Development:
The small but rather yolky eggs are fertilised in the ovaries or gonoduct. During their passage through the gonoduct the eggs are covered with the secretion of the eosinophilous glands. This secretion forms a loose membrane over the eggs and embryos and is drawn out into a stalk of attachment.
These stalks adhere to the embroyophore or brood chamber which is the name given to the anal cone.
During brooding of the embryos, the wall of the brood chamber becomes thick and filled with food inclusions that are later ingested by the embryos. As new eggs issue from the gonopore, the already attached embryos are pushed forward so that there is a regular succession of stages in the brood chamber. Development proceeds in the brood chamber to the production of free-swimming larva.
10. Affinities of Entoprocta:
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According to Hyman (1951) the union of the Entoprocta and the Ectoprocta cannot be maintained although supported by Marcus (1939). The insuperable difficulty in the way of this union is the pseudo coelomate nature of the body cavity of the Entoprocta, whereas the Ectoprocta are typical coelomate animals.
The Entoprocta are, thus, of a much lower grade of structure than are the Ectoprocta and cannot be united with them in the same phylum. The Entoprocta was linked with Ectoprocta because of some common features, viz., a crown of ciliated tentacles and a looped digestive tract.
However, the occurrence of tentacular structures on the distal end is common in sessile animals as a food catching device.
As regards other anatomical features the two groups are quite dissimilar. The larval similarities of the two groups seem the best ground for postulating relationship. But in their further development the two larvae differ altogether. Thus, it is presumed that the similarities between the larvae are because of the pelagic habits of both.
The Entoprocta was also related with the annelid-molluscan types because of the similarity of the trochophore larva but it has been observed that the entoproct larva bear no great resemblance to a trochophore either superficially or as to structural details. The formation of coelom also takes place in a different manner in both the groups. The body cavity of Entoprocta is of a pseudocoelomate type.
Among the pseudo coelomate groups the Rotifera come close to the Entoprocta. The loxosomatid resembles the collothecacean rotifier in the following:
(1) Both have a trumpet-shaped body with the free surface bordered by ciliated or bristle bearing projections that are simple extensions of the body wall.
(2) The stalk in both is a post-embryonic outgrowth provided with pedal glands at least temporarily.
(3) In both the mouth lies within the crown of tentaculate projections and in both the digestive tract makes a decided curve. In the rotifers this curvature of the digestive tract begins among the sessile forms and gets more and more pronounced with greater assumption of the sessile life.
Although even in the most evolved of the collothecacean rotifers the anus still remains outside the coronal projections, it is clearly getting nearer and nearer to the mouth. However, no great importance can be placed on this point because a looped digestive tract is a common feature of sessile animals.
Besides this, the parts of the digestive tract are similar in both the groups and it may be noted that the mastax, of which no trace occurs in the Entoprocta, is in the process of degeneration among the collothecacean rotifers.
(4) Both groups have protonephridia of flame-bulb type.
(5) A pair of eyes is present in both loxosomatid and collothecacean young ones.
(6) The ciliary rim of the ventral surface of the entoproct larva is probably a remnant of an originally completely ciliated ventral surface, as is the rotifer corona.
(7) The pair of preoral organ of the entoproct larva and the loxosomatid adult is homologous with the lateral antennae of rotifers. In the loxosomatid adults they are situated towards the free end of the calyx. In the collothecacean rotifers too they have migrated to an anterior position. Therefore, it can be easily concluded that the Entoprocta are pseudo coelomate animals with nearest affinities with the rotifers.
