Hemichordata
.
Acorn Worm, common name for simple, wormlike marine animals in the hemichordate phylum. They are of special interest because of their close relationship to chordates. This connection is evident in the adult anatomy. Some representative acorn worms have gill slits, traces of a supporting structure resembling a notochord, and a tubular nerve cord, which are features characteristic of vertebrates. The larval stages of acorn worms, however, are very much like those of echinoderms such as starfish, indicating a remote common ancestry of echinoderms and vertebrates.
The hemichordates are divided into two classes comprising about 50 species. The first class, the acorn worms, consists of animals that average 10 cm (4 in) in length, although some species may be up to 1.5 m (up to 5 ft) long. They construct burrows, commonly U-shaped, in sand of shallow seafloors using an extendable, muscular proboscis attached to a thick collar that resembles an acorn—hence the name. They secrete a slime that collects food particles on the proboscis and collar, but some species filter sediments and sand through a complicated pharynx with many gill slits. The second class consists of small, usually colonial animals of the deep sea. They are not worm-shaped but stout, and they usually construct tubes. Food is captured by tentacles that project from the tube. The body is much simplified, and gill slits are reduced to one pair or none.
See also Balanoglossus.
Scientific classification: Acorn worms make up the class Enteropneusta in the phylum Hemichordata. The other class of hemichordates is Pterobranchia.
The hemichordates are divided into two classes comprising about 50 species. The first class, the acorn worms, consists of animals that average 10 cm (4 in) in length, although some species may be up to 1.5 m (up to 5 ft) long. They construct burrows, commonly U-shaped, in sand of shallow seafloors using an extendable, muscular proboscis attached to a thick collar that resembles an acorn—hence the name. They secrete a slime that collects food particles on the proboscis and collar, but some species filter sediments and sand through a complicated pharynx with many gill slits. The second class consists of small, usually colonial animals of the deep sea. They are not worm-shaped but stout, and they usually construct tubes. Food is captured by tentacles that project from the tube. The body is much simplified, and gill slits are reduced to one pair or none.
See also Balanoglossus.
Scientific classification: Acorn worms make up the class Enteropneusta in the phylum Hemichordata. The other class of hemichordates is Pterobranchia.
Comments