[11] The body length normally range from 2 centimetres (0.79 in) to 2.5 metres (8 ft 2 in) (Balanoglossus gigas),[12] but one species, Meioglossus psammophilus, only reach 0.6 millimetres (0.024 in).

[14] Most acorn worms range from 9 to 45 centimetres (3.5 to 17.7 in) in length, with the largest species, Balanoglossus gigas, reaching 1.5 metres (5 ft) or more.

The body is made up of three main parts: an acorn-shaped proboscis, a short fleshy collar that lies behind it, and a long, worm-like trunk.

Unlike the hearts of most other animals, however, this structure is a closed fluid-filled vesicle whose interior does not connect directly to the blood system.

[16] Acorn worms continually form new gill slits as they grow in size, with some older individuals of species like Balanoglossus aurantiacus having more than a hundred on each side.

[19] Each slit consists of a branchial chamber opening to the pharynx through a U-shaped cleft and to the exterior through a dorso-lateral pore (see diagram below).

[citation needed] Acorn worms have gill-like structures that they use for breathing, similar to the gills of primitive fish.

An interesting trait is that its three-section body plan is no longer present in the vertebrates, except for the anatomy of the frontal neural tube, later developed into a brain which is divided into three main parts.

[citation needed] Studies have shown that the gene expression in the embryo share three of the same signaling centers that shape the brains of all vertebrates, but instead of taking part in the formation of their neural system,[21] they are controlling the development of the different body regions.

[23][24] Pterobranchia Stereobalanus Harrimaniidae Spengeliidae Torquaratoridae Ptychoderidae Acorn worms are rarely seen by humans because of their lifestyle.

To obtain food, many acorn worms swallow sand or mud that contains organic matter and microorganisms in the manner of earthworms (this is known as deposit feeding).

At low tide, they stick out their rear ends at the surface and excrete coils of processed sediments (casts).

Another method that some acorn worms use to obtain food is to collect suspended particles of organic matter and microbes from the water.

[17] Acorn worms are dioecious, having separate biological sexes, although at least some species are also capable of asexual reproduction in the form of fragmentation.

[25] They have paired gonads, which lie close to the pharynx and release the gametes through a small pore near to the gill slits.