Hemichordata: The Half–Chordates in Animal Kingdom!
Introduction:
The name Hemichordata comes from the Greek words “hemi” meaning “half” and “chorde” meaning “string”. So, Hemichordata means “half-chordate”.
Why are they called half Chordate?
In the head region the buccal diverticulum also called stomochord (looks like a rudimentary notochord) is located in the anterior part of the animal, within the proboscis. But it is not a notochord originally. It originates as an outgrowth from the roof of the buccal cavity projecting into the proboscis.
Structure:
Despite the name, Hemichordates are not considered true chordates. They share some characteristics with chordates, such as gill slits and a dorsal nerve cord, but they lack other key features like a notochord and a post-anal tail.
Origin and Evolutionary Significance
Balanoglossus is a fascinating marine worm and the features make it crucial for understanding the evolutionary transition between invertebrates and chordates. The tornaria larva provides crucial evidence for phylogenetic relationship with deuterostome of echinoderms. Hemichordates occupy a key phylogenetic position, bridging the gap between invertebrates and chordates.
Balanoglossus: General and Specific Characters and Classification
General Characters
- Marine living: Exclusively marine, living in burrows in soft sediments.
- Bilateral Symmetry: Worm-like Body, elongated, soft-bodied, and bilaterally symmetrical.
- Triploblastic: Three germ layers appear during the embryonic stage
- Enterocoelomates: Coelom develop from endoderm (primary gut)
- Filter feeders utilise cilia on the proboscis to collect food particles.
- Open type of Circulatory System: Blood vessels and a heart are present, but blood does not circulate in the blood vessels.
- Respiration by Gills: Gill slits located in the trunk facilitate gas exchange.
- Nervous System: Simple, with a nerve net and a dorsal nerve cord in the collar.
- Sexual Reproduction: Separate sexes (unisexuals), external fertilization (in the water).
- Deuterostome Development: Blastopore developed as anus.
Specific Characters
- Proboscis: Cone-shaped, muscular, and ciliated, used for burrowing, food collection, and sometimes locomotion. Contains the buccal diverticulum (stomochord).
- Collar: Short, cylindrical, contains the nerve cord and connects the proboscis to the trunk.
- Trunk: Elongated, contains the majority of internal organs, including the gill slits, digestive system, and gonads.
- U-shaped Gut: The digestive system is U-shaped, with the mouth and anus located near each other.
- Tornaria Larva: Free-swimming, ciliated larva with a characteristic shape, crucial for dispersal.
Classification
The Tornaria Larva of Balanoglossus: Importance, Affinities, and Evolutionary Significance
The tornaria larva of Balanoglossus, a hemichordate, is a free-swimming planktonic larva that plays a crucial role in the life cycle of these acorn worms and provides valuable insights into the evolutionary relationships among deuterostomes.
Importance
- Dispersal: The tornaria larva allows Balanoglossus to disperse to new areas, as adult acorn worms are relatively sedentary, living in burrows in the seabed.
- Development: Studying the development of the tornaria larva provides information about the embryology and larval morphology of hemichordates.
- Phylogenetic Studies: The tornaria larva’s features are used in phylogenetic analyses to understand the evolutionary relationships between hemichordates, echinoderms, and chordates.
Affinities
The tornaria larva exhibits similarities with the larvae of other deuterostomes, particularly echinoderms:
- Bipinnaria Larva: The tornaria larva shares a striking resemblance with the bipinnaria larva of sea stars (starfish). Both larvae have a similar shape, ciliated bands for locomotion, and a digestive system. This similarity suggests a common ancestral larval form for deuterostomes.
- Dipleurula Larva: The tornaria larva is also considered a type of dipleurula larva, a hypothetical ancestral larval form that is thought to have given rise to the larvae of echinoderms and hemichordates.
Q: What is “neotany” and “Paedogenesis”? Explain its significance in the evolution.
Neoteny is a fascinating biological phenomenon where an animal reaches sexual maturity while retaining juvenile (larval) characteristics without undergoing the metamorphosis.
Here are some examples of neoteny and paedogenesis:
- Axolotl: The axolotl is a salamander that exhibits both neoteny and paedogenesis.It retains its external gills and aquatic lifestyle throughout its life and it also becomes sexually mature while still in its larval form.
- Insects: Many insects exhibit paedogenesis, where the larval stage is capable of reproduction. This is particularly common in species with short life cycles, where it allows for rapid population growth.
- Aphids: Some aphids are wingless and remain in their larval form throughout their lives. They are also parasitic on plants, and they reproduce by parthenogenesis.
- Termites: Some termite queens are wingless and remain in their larval form throughout their lives. They are responsible for laying eggs and maintaining the colony.
Significance of Neoteny:
- Adaptation to specific environments: In some cases, neoteny can be an adaptation to a specific environment where the larval form is better suited for survival than the adult form. For example, if the aquatic environment is stable and资源-rich, remaining in the larval stage might be advantageous.
- Evolutionary innovation: Neoteny can lead to significant evolutionary changes. By retaining larval features, animals can explore new evolutionary pathways and adapt to different ecological niches.
- Paedomorphosis: Neoteny is a type of paedomorphosis, which is the retention of juvenile traits in adults and reaches sexual maturity.
Echinoderm Theory: Origin of Hemichordates from Echinoderms.
The larvae of some echinoderms share similarities with the tornaria larva of hemichordates, leading to theories about a common deuterostome ancestor, which reveals the origin of Hemichordates from Echinoderms through Neoteny and peadogenesis.
Hemichordate theory: Origin of Chordates from Hemichordates:
Neoteny and peadogenesis in a tornaria-like ancestor might have played a role in the evolution of chordates. The idea is that a neotenic tornaria retaining its free-swimming larval features and showing some similarities with tadpole larvae of urochordata could have eventually given rise to the tadpole larva of urochordates and, ultimately, vertebrates.