Phylum Hemichordata

There are distinct animals with distinct structures and forms. Till now, a million species of animals have been introduced, and the urge for classification becomes all the more essential. The classification also assists in allowing a systematic position for newly introduced species. Animals have definite shapes and sizes. They are unbranched except for the sponges. Animal organs are generally internal, e.g., liver, heart, kidney, lungs, brain, stomach, etc. 

The growth of an animal’s body is limited, and it stops long before death. The growth regions are not localized. They can move bodily from one place to another. They have sense organs, nervous systems, and excretory organs. There are different Phyla such as Porifera, Cnidaria, Ctenophora, Platyhelminthes, Nematode, Annelida, Arthropoda, Mollusca, Echinodermata, hemichordate, and Chordata. Let us have an overview of Phylum Hemichordata.

Hemichordata

Due to some similarities with chordates, some workers (Bateson) considered this group as a subphylum of the phylum Chordata, representing its lowest group. But, on grounds of its general organization, some recent scientists, like Van der Horst, Dawydoff, Marcus, and Hyman, have given it the status of an independent invertebrate phylum. The name “Hemichordata” is, however, retained for the group. Tornaria larva of Balanoglossus was first described by J. Muller.

Distinguishing Features of Hemichordata

1. They are mostly tubiculous living in burrows and are exclusively marine.
2. The body is worm-like and is divided into proboscis, collar, and trunk. 
3. The body wall is with a single-layered epidermis.
4. The hemichordates are enterococcus.
5. Stomochord is a hollow protuberance, that arises from the roof of the buccal cavity, called the ‘buccal diverticulum’. It is present in the proboscis.
6. The digestive Tract is complete.
7. They are one to several pairs when gill slits are present. Gill slits are dorsal in position. They are lateral in chordates. Respiration takes place through the gills.
8. The blood Vascular System is an open type, usually with a contractile heart vesicle and two longitudinal vessels, one dorsal and one ventral, interconnected by lateral vessels and sinuses. Blood is colorless.
9. The excretory organ is a single glomerulus situated in the proboscis, hence called the proboscis gland.
10. The earliest nervous system consists mainly of an intra-epidermal nerve plexus.
11. Sensory cells of the epidermis act as sense organs.
12. Sexes are separate. Reproduction is mostly sexual. Gonads are one too many pairs. Fertilization is external.
13.  Except in some forms, development is mostly indirect through a free-swimming tornaria larva.

Classification of Hemichordata

Phylum Hemichordata is divided into four classes:

Enteropneusta

• This class contains acorn worms.
• They have a vermiform body and are located on sandy beaches near seas in warm climates. 
• This class contains about 70 species. 
• They are known as acorn worms because of their proboscis and collar appearance.

Pterobranchia

• They live in deep sea waters and have tube-dwelling. 
• They attach to other organisms for their survival, as they are bottom dwellers. 
• This class contains about 20 species.

Order 1 Cephalodiscida

• Single paired gill slits.
• Single-paired gonads.
• Several individuals live unconnected in a common jelly-like house.
• Examples: Cephalodiscus and Atubaria.

Order 2 Rhabdopleurida

• Single gonad is present.
• Gill slits are present.
• Colonial, zooids attached by a stolon.
• Example: Rhabdopleura (single genus).

Planctosphaeroidea

• Planctosphaeroidea is known only from a few floating larvae.
• The L-shaped alimentary canal.
• Larval body is covered by extensively branched ciliary bands.

Graptolita

• These are extinct colonial hemichordates, mainly known from the fossil structures of their tubes.
• Each animal is housed in a zooid. These were plentiful in the Ordovician and Silurian periods.
• Example: Dendrograptus.

Examples

Balanoglossus (acorn or tongue worm), Saccoglossus (very much similar to Balanoglossus in habits, habitat, and structure), Cephalodiscus.

Balanoglossus

• Balanoglossus is an ocean-dwelling acorn worm genus of zoological significance because, being a Hemichordate, it is an “evolutionary link” between invertebrates and vertebrates. 
• Balanoglossus is a deuterostome, and look like the Ascidians or sea squirts, in that it has branchial openings, or “gill slits”.
• It has a notochord in the superior part of the body and has no nerve chord. It does have a stomochord, which is a gut chord enclosed by the collar.
• The heads of balanoglossus may be as small as 2.5 mm (1/10 in) or as large as 5 mm (1/5 in).

Saccoglossus

• Saccoglossus is a genus of acorn worms.
•  With 18 species, it is the largest genus in this class.
• This genus is marked mainly by the concentric rings of muscle fibers in the proboscis. 
• Many Saccoglossus can be found in coastal mud and sand habitat, frequently near bays.

Why Hemichordates are not placed under Chordates?

Hemichordates don’t have a notochord or a post-anal tail in their body, which is a differentiating feature in the chordates. Earlier, the hemichordates were placed among the chordates but due to the absence of any type of backbone or chord, these were removed from the group.

The hemichordates are marine animals and have triploblastic bodies. They are also known as acorn worms.

FAQs on Hemichordata

Question 1: Give two features of Hemichordata.

Answer:

• They are mostly tubiculous living in burrows and are exclusively marine.
• The body is worm-like and is divisible into proboscis, collar, and trunk. 

Question 2: What are the examples of Hemichordata?

Answer:

Balanoglossus, Saccoglossus, and Cephalodiscus are examples of Hemichordata.

Question 3: What is the difference between chordates and non-chordates?

Answer:

The main difference between chordates and non-chordates is that chordates have a spinal cord or backbone in their body structure whereas non-chordates are without notochord.

Question 4: What is the economic importance of Hemichordata?

Answer:

The hemichordates are most important as they provide information about the origin of chordates deuterostomes, and bilateral animals.

Question 5: Name three classes of Hemichordata.

Answer:

Enteropneusta, Pterobranchia, and Planctosphaeroidea are three classes of Hemichordata.