Phylum Hemichordata- Characteristics, classification, examples

Hemichordata Definition

Hemichordata (Gr., hemi, half; chorde, cord) means they are ‘half’ of ‘part’ chordates, a fact that is disputed. They are usually vermiform, solitary, or colonial enterocoelous coelomate animals with an intra-epidermal nervous system and a pre-oral with or without gill-slits and without typical nephridia.

Phylum Hemichordata Characteristics

1. They are exclusively marine, solitary or colonial, mostly tubicolous.
2. Their body is soft, fragile, vermiform, unsegmented, bilaterally symmetrical, and triploblastic.
3. The body is divisible into proboscis, collar, and trunk.
4. Body wall with a single-layered epidermis with mucous glands. No dermis.
5. Coelom enterocoelous, divisible into protocoel, mesocoel and metacoel.
6. Buccal diverticulum, earlier considered as the notochord, present in the proboscis.
7. The alimentary canal is a straight, complete, or U-shaped tube.
8. Dorso-lateral pharyngeal gill-slits, when present, one to several pairs. Ciliary filter feeders.
9. Simple, open, and well-developed circulatory system, including a dorsal heart and 2 longitudinal vessels, one dorsal and one ventral.
10. Excretion by a single glomerulus situated in the proboscis.
11. Primitive type nervous system consisting mainly of subepidermal nerve plexus. Dorsal collar nerve cord hollow.
12. Reproduction is mainly sexual. Sexes usually separate. Gonads one to several pairs.
13. External fertilization in seawater.
14. Development direct or indirect with a free-swimming tornaria larva.
15. Examples: Balanoglossus, Saccoglossus, Rhabdopleura, Atubaria, Dendrograptus.

Read Also: Phylum Mollusca- characteristics, classification, examples

Image Source: Luis Fernández García, Necrophorus, and Spengel, Johann Wilhelm.

Phylum Hemichordata Classification

Hemichordata includes about 80 known species and has been divided into 4 classes.

Class 1- Enteropneusta (Gr., enteron, gut +pneustos, breathed)

• Commonly called “acorn” or “tongue worm”.
• Solitary, free-swimming, or burrowing animals.
• Elongated, vermiform body with no stalk.
• The body consists of the proboscis, collar, and trunk; collar without tentaculated arms(lophophore).
• Proboscis cylindrical and tapering.
• Straight alimentary canal; mouth and anus at opposite ends. Filter feeding.
• Numerous pairs of U-shaped gill-slits.
• 2 pairs of hepatic caeca present in the middle of the trunk.
• Sexes separate. Gonads numerous, scan-like.
• Development with or without tornaria larva.
• Asexual reproduction is lacking.
• Examples: Balanoglossus, Saccoglossus, Protoglossus, Ptychodera.

Class 2. Pterobranchia (Gr., pteron, feather +branchion, gill)

• Sedentary, solitary or colonial, sessile, and tubicolous.
• Living inside secreted chitinous tubes.
• Body short, compact, with stalk for attachment.
• Proboscis with ciliated tentacles to produce ciliary feeding currents of water.
• Collar bearing ciliated arms(lophophore).
• One pair of gill-slits or none, never U-shaped.
• U-shaped alimentary canal. Anus dorsal lying near the mouth.
• Separate or united sexes. Gonads single or one pair.
• Development direct, may or may not include a free-swimming larval stage.
• Asexual reproduction by budding in some form.

Order 1. Rhabdopleurida

• Colonial, zooids connected by a stolon.
• Collar with 2 tentaculated arms.
• No gill-slits.
• Single gonad.
• Example: single genus Rhabdopleura.

Order 2. Cephalodiscida

• Solitary or several zooids living unconnected in a common gelatinous case.
• Collar with several tentaculated arms.
• Single pair of gill-slits present.
• Single pair of gonads present.
• Examples: Cephalodiscus, Atubaria.

Class 3. Planctosphaeroidea

• Class is represented by a few small, rounded, transparent, and pelagic larvae, supposed to be specialized tornaria of some unknown hemichordate termed Planctosphaera pelagica.
• Larval body covered by extensively branched ciliary bands.
• The alimentary canal is L-shaped.

Class 4. Graptolite

• Extinct colonial hemichordates, mainly known from fossil structures of their tubes.
• These are abundant in the Ordovician and Silurian periods.
• Each animal is housed in a zooid.
• Their tubular chitinous skeleton and colonial habits show an affinity with Rhabdopleura.
• Example: Dendrograptus.

References

1. Kotpal RL. 2017. Modern Text Book of Zoology- Invertebrates. 11th Edition. Rastogi Publications.
2. Jordan EL and Verma PS. 2018. Invertebrate Zoology. 14th Edition. S Chand Publishing.