Hemichordata

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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.

Types of Invertebrates

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Arthropoda -> Largest and most diverse invertebrate phylum characterized by animals with jointed limbs, a segmented body, and an exoskeleton made of chitin. Arthropods are abundant and successful in almost all habitats. The exoskeleton is divided into plates that enhance flexibility and movement. Periodic molting of the exoskeleton permits growth. Arthropods have a complex brain and nervous system. Many arthropods have a compound eye made up of numerous light-sensitive parts. Reproduction is sexual. – Examples: Ants, beetles, butterflies, lobsters, shrimp, crabs, scorpions, spiders, ticks

Ectoprocta or Bryozoa -> Small, mostly marine animals that resemble simplified horseshoe worms, with a lophophore surrounding the mouth. They live in colonies, attaching to the sea bottom or seaweed. Bryozoans are hermaphroditic and reproduce asexually (by budding) and sexually. – Examples: Moss animals

Entoprocta or Kamptozoa -> Small, mostly marine animals with a globular body mounted on a stalk. A lophophore surrounds both the mouth and anus. Entoprocts live in colonies. They reproduce both asexually and sexually. – Examples: Urnatella, Pedicellina

Brachiopoda -> Marine animals that resemble clams except that their shells form on the top and bottom of the animal, while clam shells form on the left and right of the animal. The shell attaches to the ocean bottom, rocks, or other objects by means of a cordlike stalk. Brachiopods reproduce sexually. – Examples: Lamp shells

Echinodermata -> Marine animals distinguished by their radial symmetry in which the body can be divided into five parts arranged around a central axis. They have internal skeletons composed of calcareous ossicles with projecting spines that give the body surface a bumpy appearance. They use body appendages called processes for feeding and locomotion. Reproduction is sexual and some have a larval form. – Examples: Sea stars, brittle stars, sea urchins, sand dollars, sea cucumbers

Cycliophora -> Discovered in 1995, these parasites live in the mouthparts of certain lobsters. They have a characteristic saclike body with a bell-shaped mouth structure called a buccal funnel. Only one species has been identified, and it displays alternation of generations, with both asexual and sexual components of the life cycle. – Example: Single species identified as Symbion pandora

Loricifera -> Marine sediment dwellers with a protective structure called a lorica, consisting of plates that surround the animal's trunk. The mouth and head retract into the lorica for protection. Reproduction is sexual and little is known about the life cycle, although a larval form has been identified. – Examples: Nanaloricus mysticus

Onychophora -> Terrestrial, caterpillar-like animals found only in the tropics and southern hemisphere. The soft body is covered by a flexible cuticle that periodically molts. The head region contains a pair of antennae and clawlike mandibles. They have from 14 to 43 pairs of legs, depending upon the species and gender. Reproduction is sexual. – Examples: Velvet worms

Chaetognatha -> Torpedo-shaped marine animals with fins that enable them to swim with rapid, dartlike movements as well as gliding and floating motions. Movable hooks on their heads are used to capture prey. These animals are hermaphrodites and reproduction is sexual. – Examples: Arrow worms

Hemichordata -> Simple, wormlike marine animals with a primitive notochord and a system of gills. They use a distinctive proboscis to capture food and also to aid in locomotion. Reproduction is sexual and some have a larval form that resembles the larvae of echinoderms. – Examples: Acorn worms