They are marine worm-like animals having about 90 known species of worldwide distribution, all of which are benthic (living on the sea floor) in their adult form. The smallest species are only a few millimetres long and the largest, Balanoglossusgigas, can reach lengths of 1.5 metres. All known species feed on organic matter either as filter feeders or as substrate eaters, feeding on microscopic algae, diatoms, and nutrients they scrape from particles of sand and mud or collect from the water.
Hemichordates have a bilaterally symmetrical body with true coelom, a tube like gut, straight or U-shaped, with a terminal anus. Body is divided into three parts, a proboscis, a collar and a trunk. Nervous system is normally diffused. They possess glomerulus in proboscis coelom as an excretory organ. Reproduction is normally sexual but pterobranchs also reproduce by budding. The first part of body is called the Protosome, which is modified as proboscis and that gives the Hemichordates their common name of Acorn Worms. The protosome is followed by a collar which bears tentacles in the Pterobranchia but not in the Enteropneusta. Behind collar is a trunk which contains the digestive and reproductive organs.
The phylum Hemichordata is divided into three living classes, namely, the Pterobranchia,Enteropneusta and Planktosphaeroidea, and a fossil group, Graptolithina. Enteropneusts or acorn worms are solitary, worm-like, bilaterally symmetrical animals that are often brilliantly coloured. They are known as acorn worms because of the appearance of the proboscis and collar. Pterobranchs are minute, sedentary, colonial and tube-building forms.
The Enteropneusts, with more than 70 species, comprise the majority of the Hemichordates. Examples: Balanoglossus and Saccoglossus. They live in burrows in the sand or under rocks, in both shallow and deeper waters. Feeding is either by filter feeding or substrate feeding. The skin is covered with cilia as well as glands that secrete mucous. Some produce a bromide compound that gives them a medicinal smell and might protect them from bacteria and predators. Acorn worms typically live in burrows on the sea-bed, from the shoreline down to a depth of 3,000 m. The worms lie there with the proboscis often sticking out of one opening in the burrow.
The largest species is Balanoglossus gigas of Brazil, an acorn worm that reaches 4.9 ft in length and lives in long burrows that stretch over more than 9.8 ft. Body colour is green, and they frequently emit a pungent odour. The proboscis is assisted by the collar in burrowing. By the waves of contraction executed by the proboscis accompanied by inflation of the collar, body is pushed into mud, sometimes with marvellous rapidity. Trunk may attain a great length; one or two feet, or even more, and is also muscular, but the trunk muscles are of subordinate importance in locomotion, serving to promote the peristaltic contractions of the body. Substrate eaters like Balanoglossusclavigerus from the Mediterranean are generally larger than filter feeders. They consume large amounts of mud and or sand and digest the organic matter within it. They deposit their wastes on the surface much like earthworm castes. The acorn worm’s body is cylindrical and made up of three main parts: the acorn-shaped proboscis, a short fleshy collar that lies behind it and a long trunk. The worm’s mouth is located between the proboscis and collar. Other species of Balanoglossus are as follows: Balanoglossus apertus; B. aurantiacus; B. australiensis; B. biminiensis; B. capensis; B. carnosus; B. clavigerus; B. gigas; B. jamaicensis; B. misakiensis; B. natalensis; B. numeensis; B. occidentalis; B. proterogonius; B. salmoneus; B. stephensoni.
Filter feeders have mucous secreting glands and cilia on their proboscis. The proboscis is held out of the burrow entrance and organic particles are caught in the mucous which is swept to the mouth by the beating of the cilia. The digestive system is a tube like gut ending in a terminal anus. Behind the mouth is a buccal cavity which leads into a pharynx having gill slits for respiration. The Pharynx leads into an oesophagus which in turn leads to an intestine which is the main site of digestion. The intestine ultimately opens into a terminal anus.
In most species of Balanoglossus each gill-slit may open into its own atrial chamber or gill-pouch; this in its turn opens to the exterior. There are, therefore, as many gill-pouches as there are gill-slits. The gill-pores occur on each side of the dorsal aspect of the worm in longitudinal series at the base of a shallow groove, the branchial groove. Excretion is mainly accomplished by a structure peculiar to the Enteropneusta called the glomerulus, a vascular complex placed on either side of the anterior portion of the stomochord, projecting into the proboscis-coelom. Gaseous exchange occurs over the whole body surface as well as in the pharyngeal slits. Acorn worms breathe by drawing in oxygenated water through their mouth. The water then flows out the animal’s gills, which are on its trunk.
The vascular system itself is quite peculiar, consisting of lacunae and channels without endothelium. Central sinus lies over the stomochord, and is surrounded on three sides by a closed vesicle of contractile walls, called the pericardium. By the pulsation of the pericardial vesicle the blood is driven into the glomerulus. The blood is colourless and has no respiratory pigments. The heart consists of a sinus located in proboscis but the blood does not actually enter the heart so it is not a heart in the strict sense. The blood passes through two longitudinal blood vessels, dorsal and ventral, and a series of sinuses.
The nervous system is a sub-epidermal diffused net that is thickened in the mid-dorsal and midventral regions, and hollow nerve cord in the collar region where there are giant nerve fibres, whose function is poorly understood.