Echinodermata: Overview | Zoology Optional Notes for UPSC PDF Download

Phylum Echinodermata: Characteristics and Taxonomy

Introduction

Etymology

Diagnostic Features of Phylum Echinodermata

1. Exclusively Marine: Echinoderms are exclusively marine organisms.
2. Radial Symmetry: Adults display radial and five-rayed symmetry, whereas larvae exhibit bilateral symmetry.
3. Oral and Aboral Surfaces: The body can be divided into oral and aboral surfaces but lacks a differentiated head.
4. Calcareous Ossicles: The body surface is covered with calcareous ossicles or plates, often featuring tubercles or spines.
5. Ambulacra: Underneath the body, five grooves, called ambulacra, radiate from the mouth to the tip of the arms, with inter-ambulacral spaces in between.
6. Coelomic Cavities: Echinoderms possess primarily oligomeric, tripartite paired enterocoelic body cavities, mainly metacoels (or "somatocoels"). In some forms, the body cavities are schizocoelic.
7. Digestive System: The digestive canal is mostly a coiled tube, with the anus located on the aboral side.
8. Water Vascular System: A characteristic water vascular system, primarily derived from the left mesocoel (or "left hydrocoel") and partly from the left protocoel (or "left exocoel"), is present. It serves multiple functions.
9. Tube Feet: Echinoderms have tubular, contractile tube feet (or podia) used for locomotion and/or feeding.
10. Circulatory System: The haemal circulatory system is poorly defined.
11. Respiratory and Excretory Systems: Echinoderms do not possess distinct respiratory or excretory systems.
12. Nervous System: The nervous system is sub-epidermal and appears as a circumoesophageal ring, from which diffused nerves extend along each ambulacrum.
13. Reproduction: Echinoderms are typically gonochoristic, with sexual reproduction. Gonads are simple in structure, and fertilization is external.
14. Development: Development in Echinoderms is deuterostomatous, characterized by indirect development via ciliated, bilaterally symmetrical, free-swimming larval forms.

Scheme of Phylum Echinodermata Classification

The classification of Phylum Echinodermata has evolved over time, with Jacob Klein coining the term "Echinodermata" in 1734. Jean-Baptiste Lamarck further classified these animals in 1801. The classification presented here is based on the plan outlined by Ruppert and Barnes in 1994.

Systematic Overview of Phylum Echinodermata

Subphylum Homalozoa (Extinct)

• Paleozoic echinoderms that lack evidence of radial symmetry.

Subphylum Crinozoa

• Stalked and free-moving forms with the oral side directed upwards.
• Arms are branched, with 10 to 200 or more branches, each bearing pinnules.
• Ambulacral grooves radiate from the mouth and extend to the tip of the pinnules.
• Absence of ampullae for operating the podia.
• Presence of a non-contractile aboral stalk, often with cirri, terminating in a disc for attachment.
• Heavily calcified plates in the arms, body, and stalk.

Subclass Inadunta (Extinct)

• Stalked Paleozoic crinoids with or without cirri, some without pinnules.

Subclass Articulata

• Comprises both extinct and living crinoids.
• Examples include Antedon (stalkless), Metacrinus (with cirri), and Rhizocrinus (without cirri).

Subphylum Asterozoa

• Arms are not distinctly separated from the central disc.
• Open ambulacral grooves with a large coelomic cavity in the arms.
• Each ambulacral groove contains two or four rows of tube feet, with or without suckers.
• Anus and madreporite are located on the aboral surface.
• Examples include Asterias, Astropecten, Heliaster, Platyasterias, and Asterina.

Class Ophiuroidea (Snake-like)

• Arms are sharply separated from the central disc.
• Absence of ambulacral grooves; arms are filled with vertebral ossicles.
• No spacious prolongations of the coelom into the arms.
• Mouth and madreporite are situated on the oral surface.
• Anus is absent.
• Examples include Ophiura, Ophiothrix, Ophioderma, Ophionereis, and Asteronyx.

Class Concentricycloidea (Concentric Ring)

• Minute echinoderms with a disc-shaped body.
• Aboral surface covered with plate-like ossicles and marginal spines.
• Water vascular system features two ring canals with tube feet emerging from the outer one.
• Absence of arms, but tube feet passage between ossicles is similar to stelleroidea.
• Examples include Xyloplax medusiformis and Xyloplax turnerae.

Subphylum Echinozoa

• Body is more or less spherical, flattened either orally or aborally.
• Ossicles are fused to form an internal test, and movable spines are mounted on it.
• The body surface is divided into alternate ambulacral and inter-ambulacral areas, despite the absence of ambulacral grooves.
• Tube feet possess suckers.
• Subclass Perischoechinoidea includes the order Cidaroida, featuring widely separated primary spines and small secondary spines, with no gills.
• Examples: Cidaris, Notocidaris, and Eucidaris.
• Subclass Euechinoidea encompasses species with rigid or flexible, flattened or round to oval tests. They may have hollow or solid spines and typically feature Aristotle's lantern, a feeding structure.
• Examples: Echinus, Arbacia, Clypeaster, and Diadema.

Class Holothuroidea (Violent Expulsion)

• Elongated body along the oral-aboral axis.
• The body may exhibit five ambulacral areas.
• Mouth and anus located at opposite extremities.
• Oral podia are modified as tentacles, which can be simple or dendritic and may or may not have ampullae.
• Skeleton is reduced to microscopic ossicles.
• Examples: Holothuria, Cucumaria, Thyone, and Molpadia.

In summary, Phylum Echinodermata encompasses a diverse array of marine animals characterized by unique diagnostic features and a hierarchical classification system based on their distinctive characteristics and evolutionary history. The phylum includes both extant and extinct species with a range of body forms and adaptations.