Tunicata
.
Tunicate, common name for any member of a subphylum of primitive marine chordates. There are about 2000 known species. The larval stage is characterized by the presence of a notochord and a dorsal nerve cord, both of which are lacking in the adult. The adult is characterized by a layer of protective secretion called a tunic. Tunicates are common in such habitats as rocky shores but can also be found at great depths. Species known as sea squirts make up one class in the subphylum, and most tunicates belong to this class. Tunicates are important because they are an evolutionary link between invertebrates and vertebrates.
The larval stage, which generally resembles a tadpole, has a dorsal nerve cord protected by a notochord, a brain, gills, blood vessels, and a coelom. These features of tunicate larvae indicate that they are more closely related to the chordates than to the invertebrates. The free-swimming larva eventually attaches itself to a substrate and undergoes metamorphosis into the adult form, losing much of its resemblance to higher animals.
The adult is usually a sedentary animal possessing a complete digestive system, a heart and circulatory system, a gill net, and a reduced nervous system. It is covered by a tunic, which consists mainly of tunicin, a carbohydrate similar to cellulose. The vase-shaped animal has two siphons, one to allow water to enter the body cavity and the other to allow water to escape. The water passes through a filtering apparatus that captures small food particles and passes them to the stomach. The netlike branchial sac extracts oxygen from the water as it passes through. Most species are hermaphroditic (containing both male and female reproductive organs), but some free-living species, such as salps, reproduce alternately by sexual means and by asexual budding (see Alternation of Generations). Colonies may be formed by asexual reproduction, and some species form compound individuals that share a common excurrent siphon. One class of tiny, larvalike tunicates feed by means of a complicated mucous “house” that surrounds the body.
Scientific classification: Tunicates make up the subphylum Tunicata in the phylum Chordata. Sea squirts make up the class Ascidiacea, and salps make up the class Thaliacea. The larvalike tunicates that feed by means of a mucous “house” make up the class Larvacea.
Tunicate, common name for any member of a subphylum of primitive marine chordates. There are about 2000 known species. The larval stage is characterized by the presence of a notochord and a dorsal nerve cord, both of which are lacking in the adult. The adult is characterized by a layer of protective secretion called a tunic. Tunicates are common in such habitats as rocky shores but can also be found at great depths. Species known as sea squirts make up one class in the subphylum, and most tunicates belong to this class. Tunicates are important because they are an evolutionary link between invertebrates and vertebrates.
The larval stage, which generally resembles a tadpole, has a dorsal nerve cord protected by a notochord, a brain, gills, blood vessels, and a coelom. These features of tunicate larvae indicate that they are more closely related to the chordates than to the invertebrates. The free-swimming larva eventually attaches itself to a substrate and undergoes metamorphosis into the adult form, losing much of its resemblance to higher animals.
The adult is usually a sedentary animal possessing a complete digestive system, a heart and circulatory system, a gill net, and a reduced nervous system. It is covered by a tunic, which consists mainly of tunicin, a carbohydrate similar to cellulose. The vase-shaped animal has two siphons, one to allow water to enter the body cavity and the other to allow water to escape. The water passes through a filtering apparatus that captures small food particles and passes them to the stomach. The netlike branchial sac extracts oxygen from the water as it passes through. Most species are hermaphroditic (containing both male and female reproductive organs), but some free-living species, such as salps, reproduce alternately by sexual means and by asexual budding (see Alternation of Generations). Colonies may be formed by asexual reproduction, and some species form compound individuals that share a common excurrent siphon. One class of tiny, larvalike tunicates feed by means of a complicated mucous “house” that surrounds the body.
Scientific classification: Tunicates make up the subphylum Tunicata in the phylum Chordata. Sea squirts make up the class Ascidiacea, and salps make up the class Thaliacea. The larvalike tunicates that feed by means of a mucous “house” make up the class Larvacea.
Comments