Phylum Entoprocta
Phylum Entoprocta
Entoprocta (en´to-prok´ta) (Gr. entos, within, + proktos, anus) is a small phylum of about 150 species of tiny, sessile animals that superficially resemble hydroid cnidarians but have ciliated tentacles that tend to roll inward (Figure 15-21). Most entoprocts are microscopic, and none is more than 5 mm long. They are all stalked and sessile forms; some are colonial, and some are solitary. All are ciliary feeders.
With the exception of the genus Urnatella (L. urna, urn, + ellus, dim. suffix), all entoprocts are marine forms that have a wide distribution from the polar regions to the tropics. Most marine species are restricted to coastal and brackish waters and often grow on shells and algae. Some are commensals on marine annelid worms. Freshwater entoprocts occur on the underside of rocks in running water. U. gracilis is the only common freshwater species in North America (Figure 15-21A).
Form and Function
The body, or calyx, of an entoproct is cup shaped, bears a crown, or circle, of ciliated tentacles, and may be attached to a substratum by a single stalk and an attachment disc with adhesive glands, as in the solitary Loxosoma and Loxosomella (Gr. loxos, crooked, + soma, body) (Figure 15-21B), or by two or more stalks in colonial forms. Both tentacles and stalk are continuations of the body wall. The 8 to 30 tentacles making up the crown are ciliated on their lateral and inner surfaces, and each can move individually. Tentacles can roll inward to cover and protect the mouth and anus but cannot be retracted into the calyx.
Movement is usually restricted in entoprocts, but Loxosoma, which lives in the tubes of marine annelids, is quite active, moving over the annelid and its tube freely.
The gut is U-shaped and ciliated, and both the mouth and the anus open within the circle of tentacles. Entoprocts are ciliary filter feeders. Long cilia on the sides of the tentacles keep a current of water containing protozoa, diatoms, and particles of detritus moving in between the tentacles. Short cilia on the inner surfaces of the tentacles capture the food and direct it downward toward the mouth.
The body wall consists of a cuticle, cellular epidermis, and longitudinal muscles. The pseudocoel is largely filled with a gelatinous parenchyma in which is embedded a pair of protonephridia and their ducts, which unite and empty near the mouth. There is a well-developed nerve ganglion on the ventral side of the stomach, and the body surface bears sensory bristles and pits. Circulatory and respiratory organs are absent. Exchange of gases occurs through the body surface, probably much of it through the tentacles.
Some species are monoecious, some dioecious, and some appear to be protandrous; that is, the gonad at first produces sperm and later eggs. The gonoducts open within the circle of tentacles.
Fertilized eggs develop in a depression, or brood pouch, between the gonopore and the anus. Entoprocts have a modified spiral cleavage pattern with mosaic blastomeres. The embryo gastrulates by invagination. The trochophore-like larva is ciliated and free swimming. It has an apical tuft of cilia at the anterior end and a ciliated girdle around the ventral margin of the body. Eventually the larva settles to the substratum and inverts to form the adult.
Entoprocta (en´to-prok´ta) (Gr. entos, within, + proktos, anus) is a small phylum of about 150 species of tiny, sessile animals that superficially resemble hydroid cnidarians but have ciliated tentacles that tend to roll inward (Figure 15-21). Most entoprocts are microscopic, and none is more than 5 mm long. They are all stalked and sessile forms; some are colonial, and some are solitary. All are ciliary feeders.
With the exception of the genus Urnatella (L. urna, urn, + ellus, dim. suffix), all entoprocts are marine forms that have a wide distribution from the polar regions to the tropics. Most marine species are restricted to coastal and brackish waters and often grow on shells and algae. Some are commensals on marine annelid worms. Freshwater entoprocts occur on the underside of rocks in running water. U. gracilis is the only common freshwater species in North America (Figure 15-21A).
Form and Function
The body, or calyx, of an entoproct is cup shaped, bears a crown, or circle, of ciliated tentacles, and may be attached to a substratum by a single stalk and an attachment disc with adhesive glands, as in the solitary Loxosoma and Loxosomella (Gr. loxos, crooked, + soma, body) (Figure 15-21B), or by two or more stalks in colonial forms. Both tentacles and stalk are continuations of the body wall. The 8 to 30 tentacles making up the crown are ciliated on their lateral and inner surfaces, and each can move individually. Tentacles can roll inward to cover and protect the mouth and anus but cannot be retracted into the calyx.
Movement is usually restricted in entoprocts, but Loxosoma, which lives in the tubes of marine annelids, is quite active, moving over the annelid and its tube freely.
The gut is U-shaped and ciliated, and both the mouth and the anus open within the circle of tentacles. Entoprocts are ciliary filter feeders. Long cilia on the sides of the tentacles keep a current of water containing protozoa, diatoms, and particles of detritus moving in between the tentacles. Short cilia on the inner surfaces of the tentacles capture the food and direct it downward toward the mouth.
The body wall consists of a cuticle, cellular epidermis, and longitudinal muscles. The pseudocoel is largely filled with a gelatinous parenchyma in which is embedded a pair of protonephridia and their ducts, which unite and empty near the mouth. There is a well-developed nerve ganglion on the ventral side of the stomach, and the body surface bears sensory bristles and pits. Circulatory and respiratory organs are absent. Exchange of gases occurs through the body surface, probably much of it through the tentacles.
Some species are monoecious, some dioecious, and some appear to be protandrous; that is, the gonad at first produces sperm and later eggs. The gonoducts open within the circle of tentacles.
Fertilized eggs develop in a depression, or brood pouch, between the gonopore and the anus. Entoprocts have a modified spiral cleavage pattern with mosaic blastomeres. The embryo gastrulates by invagination. The trochophore-like larva is ciliated and free swimming. It has an apical tuft of cilia at the anterior end and a ciliated girdle around the ventral margin of the body. Eventually the larva settles to the substratum and inverts to form the adult.
 |
| Figure 15-21 A, Urnatella, a freshwater entoproct, forms small colonies of two or three stalks from a basal plate. B, Loxosomella, a solitary entoproct. Both solitary and colonial entoprocts can reproduce asexually by budding, as well as sexually. |
Support our developers
