Phylum Hemichordata is generally considered the sister group of the phylum Echinodermata. The animals are marine deuterostomes. They are more or less worm-like, having the ancestral tripartite body of proboscis, collar, and trunk.
The name Hemichordata is derived from the words Hemi and Chorde (Gr., hemi, half; chorde, cord or string). Thus, hemichordata means they are ‘half‘ chordates.
There are about 90 known species in the phylum Hemichordata (Barnes et al., 1993).
Definition of Hemichordata
Phylum Hemichordata is a group of marine deuterostomes having proboscis, collar, and trunk in the body with pharyngeal gill slits and intra-epidermal nervous system.
Characteristics of Hemichordata
- The animals are exclusively marine.
- They remain either solitary or form colonies.
- The body is soft, fragile (devoid of the skeleton), worm-like, triploblastic, and bilaterally symmetrical. It is divided into three parts: the proboscis, collar, and trunk.
- Epidermis is ciliated.
- Coelom is enterocoelous (i.e., arising from the wall of the embryonic gut) and divided into protocoel, mesocoel, and metacoel.
- The alimentary canal is straight or U-shaped.
- The buccal or pre-oral diverticulum, called the stomochord (earlier considered the notochord), is present in the proboscis.
- The pharyngeal gill slits are usually present and are in one to several pairs.
- Nervous system is of a diffuse and primitive type and, in some cases, has a hollow dorsal nerve cord in the collar.
- A simple and open circulatory system is present.
- The excretory organ consists of a single glomerulus.
- Reproduction is mostly sexual. Sexes are separate (gonochoristic) or united.
- Fertilisation occurs externally in the seawater.
- Presence of a free-swimming tornaria larva in the life cycle.
Examples of Hemichordata
Balanoglossus, Saccoglossus, Ptychodera, Atubaria, and Rhabdopleura.
Classification of Hemichordata
Phylum Hemichordata has been divided into four classes.
- Class 1. Enteropneusta (e.g., Balanoglossus)
- Class 2. Pterobranchia
- Order 1. Cephalodiscida (e.g., Atubaria)
- Order 2. Rhabdopleurida (e.g., Rhabdopleura)
- Class 3. Planctosphaeroidea (e.g., Planctosphaera pelagica)
- Class 4. Graptolite (e.g., Dendrograptus)
Class 1. Enteropneusta (Gr., enteron = gut; pneustos = breathed)
- A group of marine animals commonly known as acorn or tongue worms.
- These are solitary animals, either free-swimming or burrowing in marine habitats.
- Elongated, worm-like body.
- The body is composed of the proboscis, collar, and trunk.
- The collar is short and lacks tentaculated arms (lophophore).
- Proboscis is more or less conical.
- Straight alimentary canal with mouth and anus at opposite ends.
- Several pairs of U-shaped gill-slits. These gill-slits help in filter feeding and respiration.
- Two pairs of hepatic caeca are found in the middle of the trunk.
- The glomerulus is well developed.
- Reproduction occurs through both asexual and sexual methods.
- Sexes separate. Gonads are numerous and sac-like.
- Development with or without tornaria larva.
- Examples: Balanoglossus, Saccoglossus, and Pyrchodera.
Class 2. Pterobranchia (Gr., pteron = feather; branchion = gill)
- These are sedentary, tubicolous (live inside secreted chitinous tubes) animals.
- They can exit in solitary or colonial forms.
- Proboscis have ciliated tentacles, which produce ciliary feeding currents of water.
- Collar bears two or more tentaculated arms (lophophore).
- Trunk short and sac-like.
- One pair of gill-slits or none. Gill-slits are never U-shaped.
- Alimentary canal U-shaped, with the dorsal anus lying near the mouth.
- Glomerulus is poorly developed.
- Sexes are separate or united. A single or one pair of gonads are present.
- Development is usually direct. Some species exhibit a free-swimming larval stage in their life cycle.
Order 1. Cephalodiscida
- Solitary or several zooids living in a common gelatinous house. These zooids are unconnected.
- Collar with many tentaculated arms.
- One pair of gill-slits is present.
- Gonads one pair.
- Examples: Atubaria, Cephalodiscus.
Order 2. Rhabdopleurida
- Colonial, zooids are connected by a stolon.
- Collar has two tentaculated arms.
- Gill-slits absent.
- Gonad only one.
- Example: Rhabdopleura.
Class 3. Planctosphaeroidea
- Represents only some small, rounded, transparent, and pelagic larvae.
- These larvae have branching arborescent ciliated bands on their surface.
- U-shaped alimentary canal.
- Example: Planctosphaera pelagica.
Class 4. Graptolite
- These are extinct colonial hemichordates.
- Abundant in the Ordovician and Silurian periods.
- Each animal remains in a zooid.
- Example: Dendrograptus.