Phylum Hemichordata | Zoology Optional Notes for UPSC PDF Download

Introduction to Hemichordata

• Hemichordates are a small group of creatures, not well-known to most people, but they play a significant role in the study of vertebrate evolution. Although there are only a few hundred species in this phylum, their importance is highlighted by the well-known fossil record of a specific group called graptolites, which is often used to correlate rocks.
• Phylum Hemichordata includes a small group of soft-bodied, worm-like marine animals that are considered primitive relatives of chordates. These creatures are usually found living in tubes in the ocean. Hemichordates are characterized by a body that is divided into three main parts: the proboscis (a kind of snout), the collar, and the trunk.
• The name "hemichordate" means "half chordate," because these animals share some, but not all, of the typical characteristics of chordates. For example, their body and coelom (a fluid-filled body cavity) are divided into three unequal regions. Hemichordates are also dioecious, meaning that males and females cannot be distinguished from the outside, and they reproduce through external fertilization. Their development is indirect, involving a distinct larval stage. Some species of hemichordates are capable of asexual reproduction.

Characteristics of Hemichordata

• Body Structure: Hemichordates have a soft, vermiform (worm-like) body that is divided into three unequal regions: the proboscis (head region), collar (neck region), and trunk (body region). The body is covered by a thin cuticle, and the coelom (body cavity) is enterocoelous, meaning it forms from the mesoderm during development.
• Coelom: The coelom in hemichordates is also divided into three parts, corresponding to the three body regions.
• Reproduction: Hemichordates are dioecious, meaning males and females are separate and cannot be distinguished externally. They reproduce through external fertilization, and their development is indirect, involving a distinct larval stage. Some species of hemichordates, such as acorn worms and most pterobranchs, can also reproduce asexually.
• Habitat: Hemichordates are marine animals, living in various ocean habitats. Most species live in tubes, which they construct from various materials.
• Evolutionary Significance: Hemichordates are considered primitive chordates and share some characteristics with echinoderms, such as the presence of a water vascular system. Their evolutionary relationship with chordates and echinoderms is still a subject of research, but they are believed to provide important insights into the early evolution of these groups.

General Characters of Hemichordata 

• Symmetry and Habitat: Hemichordates are bilaterally symmetrical and are found exclusively in marine environments. They have a worm-like shape and are soft-bodied.
• Body Cavity: Their body cavity is a true coelom, which is formed via enterocoely.
• Body Segmentation: The body of hemichordates is divided into three sections: the proboscis, the collar, and the trunk.
• Gill Slits: They possess numerous paired gill slits.
• Digestive System: The digestive tube is complete and can be either straight or U-shaped.
• Nervous System: The nervous system is usually diffuse, but there is some variability.
• Circulatory System: The circulatory system is partially open.
• Excretory Organ: They have a glomerulus as an excretory organ.
• Reproduction: Reproduction is typically sexual, with separate sexes. However, some acorn worms can reproduce asexually. Fertilization occurs externally in seawater.
• Development: Development can be direct or indirect, with a free-swimming tornaria larva. 
• Feeding: Hemichordates feed on fine particles found in the water. 
• Etymology: The name "Hemichordata" comes from the Greek word "Hemi," meaning half, and the Latin word "Chorda," meaning chord. 

Classification of Hemichordata

The phylum Hemichordata is classified into four distinct classes:

Class 1: Enteropneusta:

• Etymology: The name comes from Greek words "enteron" (gut) and "pneustos" (breathed). 
• Habitat: These are solitary, free-swimming, or burrowing animals commonly known as "acron" or "tongue worms." 
• Physical Characteristics: Enteropneusts are worm-like creatures with multiple pharyngeal gill slits and a straight alimentary canal. 
• Development: Some species undergo development that includes a larval stage known as the tornaria larva. 
• Reproduction: They do not reproduce asexually. 
• Examples: Balanoglossus, Spengelia, Protoglossus, Saccoglossus, and others. 

Class 2: Pterobranchia:

• Etymology: The name is derived from Greek words "pteron" (feather) and "branchion" (gill). 
• Habitat: Pterobranchs are sessile animals, meaning they are attached to a substrate. 
• Alimentary Canal: Their alimentary canal is U-shaped, with the anus located dorsally (on the back side). 
• Proboscis: The proboscis of Pterobranchs is shield-like, and they have either one pair of gill slits or none at all. 
• Reproduction: Asexual reproduction occurs through budding, and the sexes are separate with one pair of gonads. 
• Examples: Rhobdopleura, Cephalodiscus, and others. 

Class 3: Planctosphairoidea:

• Description: These organisms are known only from a round transparent pelagic larva that is related to the tornaria larva. 
• Alimentary Canal: The alimentary canal of the larva is U-shaped. 
• Examples: Planctosphairoidea and related forms. 

Class 4: Graptolithina:

• Description: Graptolithina are known from fossils and are extinct colonial hemichordates. 
• Physical Characteristics: They are characterized by their tubular chitinous skeleton and colonial habits, which show an affinity with Rhobdopleura.