ENTOPROCTA – STRUCTURE, LIFE HISTORY AND AFFNITIES

INTRODUCTION

• The Entoprocta or Endoprocta are group of small sessile aquatic animals. In which the body cavity is believed to be pseudocoel. They were formerly include in the phylum Bryozoa which however are true coelomate animals.
• Members of phylum Entoprocta were known as early as 1774, when Pallas describe a colony of Pedicellina. In 1870 Bryozoa got divided into two groups.
• Endoprocta – including the genera Pedicellina, loxosoma and Urnatella in which Anus lies inside the circlet of tentacles
• Ectoprocta – includes all other known bryozoans in which anus lies outside the tentacles.
• The endoproctas are simple, solitary or colonial animals without coelom the space between guts and body wall is occupied by parenchyma with looped alimentary canal having mouth and anus located inside the circlet of tentacles. Their body is proper vase like or cup like and called calyx. It is slightly flattened laterally and contains the viscera.
• The free upper edge or rim of calyxis oval or round called lophophore which bears encircle ciliated crown of tentacles.
• Usually a calyx joints a contractile stalk or pedicle and an attached disc with adhesive gland. The stalk posses much variation and it is of taxonomical value.

STRUCTURE OF ENTOPROCTA

• Their body is divided into stalk and calyx as describe earlier

Body wall

• The structure of body wall is simple. The surface of the body except for tentacles and vestibules are covered with cuticles of varying thickness. Below it lies single layer epidermis consisting of cuboid cells and glands cells.
• A sphinter of circular fibres is present  in tentacular membrane.

Pseudocoel

• A proper body cavity is lacking. The interior of tentacles and the stalk and space between body wall and alimentary canal are filled by loose gelatinous parenchyma.

Digestive system

• The alimentary canal is u- shaped occupying most of the space inside the calyx. It includes a small funnel shaped buccal cavity, a narrow tubular oesophagus, an enlarge stomach, a narrow intestine and  a terminal rectum opening by anus.
• The mouth and anus are place close to each other inside the tentacular circlet.
• Mouth opening is surrounded by two lips upper and lower which is further continued with vestibular groove.
• Anal cone is an elevation for anal opening.
• Entoprocata are ciliary feeders and their food generally consist of Diatoms, Planktons and other microscopic organisms.
• The frontal cilia of tentacles entrap the suspended food and pass them onto the vestibular groove leading into the mouth.
• Stomach glands are believed to secrete enzymes for digestion.
• The digestive tract is mostly lined by ciliated epithelium.

Excretory system

• Each calyx have single pair of protonephridia resembling those of Annelids trocosphere. These are ciliated intracellular canals each beginning in a flame cell. The two excretory tube either separately or unite to open by nephridiopore into the vestibule or rectum.

Nervous system

• There is presence of rectangular bilobular ganglionic mass located between stomach and vestibules.
• It represents the subenteric ganglion as cerebral ganglion is lost during metamorphosis.
• Nerve radiates from ganglion to supply to the crown of tentacles, calyx wall, the stalk and the gonads.
• Tactile sensory nerve cells ending in bristles are found scattered in the epithelium.

Reproductive system

• Some are hermaphrodite while other are dioecious. Two simple rounded gonads are place between the vestibule and the stomach each with a simple gonoduct. The two gonoduct join to open by a common gonopore just behind the nephridiopore. In hermaphrodite species there are two testes and two ovaries and all four gonoducts open by single gonopore..
• In male the common sperm duct forms a swelling in a seminal vesicle for storage of ripen sperm which are flagellated.

LIFE CYCLE OF ENTOPROCTA

• Entoprocta reproduce sexually by the fusion of ova and sperm. The small and yolky eggs are fertilized in ovaries and become surrounded by membrane formed by the secretion of gonoduct. The membrane is drawn out into the stalk, by which the zygote adhere to the embryopore or  vestibular wall in front of the anal cone. The calyx surface between the gonopore and anal cone is depress into the genital recess serving the brood chamber for developing embryos into the hermaphrodite and the female.
• Cleavage is equal or spiral resulting in to the free swimming larva. It breaks through the enclosing membrane and escape from maternal brood pouch. It is known as trocophore because of its superficial resemblance with Annelids and Mollusca. It has an apical tuft of cilia at anterior end and a ciliated girdle around the ventral margin of the body. After short period of free swimming larva fixes itself by ora surface and undergo complex metamorphosis in which future calyx rotate 180 degree. The vestibules with mouth anus and developing tentacles are carried to occupy permanent position on the free surface and the larva transformed into the inverted adult form.

Asexual reproduction

• Asexual reproduction takes place by external budding from the sides of the calyx, stalk and stolon. When complete the bud get separate and attach elsewhere.  When the bud remain attach the colony formation is result.
• They also posses the great power of regeneration during the unfavourable condition.

AFFINITIES OF ENTOPROCTA

In their external appearance Ectoprocta recalls the Hyroidea from the coelenterates however they are not related to them they are readily distinguishable the presence of the cilia on tentacles and possession of long alimentary canal.

Affinities With Ectoprocta

Previously endoprocta were supposed to be closely related with Ectoprocta, but they were included in phylum Bryozoa. This relation was mainly based upon possession by both group of ciliated tentacular crown and looped digestive tract. But this kind of features are quite common in mostly all sessile animals. Distal tentacular circlet act as food catching device.

Differences

• Entoprocta are pseudocoleomate while Ectoprocta are coelomate
• Gonadoduct are present in entoprocta ehile absent in ectoprocta
• In ectoprocta anus is not outside of the body.

Affinities With Annelid And Molluscan

• The spiral determinant cleavage and similarity of trochophore show relationship of entoprocta with annelids and molluscan
• However trochophore larva differ with entoprocta by not having preoral organs and vestibular depression, shape and relation with digestive tract.

Affinities With Rotifera

They bear closer affinity with rotifer among the pseudocoleomate group.

• Body is trumpet like
• Ciliate projections are simple extension of body wall
• Stalk develop post embryonically
• Mouth lies within the crown of the tentacular projection
• Digestive tube is curved
• Protonephridia of flame bulb type occur
• Juvenile posses pair of eyes

CONCLUSION

• It is fair to conclude that nearest affinity of entoprocta is with rotifers. It is phylum of mostly sessile organism. They are the group of aquatic species majority of them are marine found in coastal area around the world except the Antarctic. Total number of species in entoprota is quite less which is about only 150. They superficially resemble cnidaria.
• Colonial entoprocta are found living on rocks, algae, shell, and under water structure preferring more solid substrates than softer sediment. They are found from intertidal areas to depth of up to 500 metres all though no definitive studies regarding the lifespan of entoprocta are currently available, feeding planktonic larvae of some species live for more than seven months.
• As very small invertebrates entoproctas are likely to prey to many large marine and fresh water animals. Known predators includes small crustaceans and mollusc, flatworms and nudibranchs.