Understanding Echinoderm Spiny Skin
Echinoderms are a diverse group of marine invertebrates found in all ocean depths, from shallow coastal waters to the deepest trenches. This unique phylum includes familiar creatures like sea stars, sea urchins, and sea cucumbers, alongside lesser-known brittle stars and crinoids.
Echinoderms have spiny skin, which is an integral part of their internal skeletal system. This endoskeleton is composed of small, interlocking plates called ossicles. These ossicles are made primarily of calcium carbonate.
The ossicles are embedded within the dermis, the connective tissue layer beneath the outer epidermis, forming a rigid or flexible framework. In many echinoderms, the spines are direct extensions of these ossicles that protrude through the skin.
Functions and Forms of Spiny Skin
The spiny skin of echinoderms serves multiple purposes, offering protection from predators and providing structural support. The hard, often sharp spines deter many threats, making echinoderms less appealing prey. Interconnected ossicles give the animal its characteristic shape and rigidity, aiding movement and support.
The appearance of spiny skin varies across different echinoderm classes. Sea urchins, for instance, are known for their long, sharp spines, which can be moved using muscles at their base. These spines provide defense and can aid in locomotion or burrowing.
Sea stars feature smaller, blunter spines or granular projections embedded in their skin, giving their surface a rough texture. Brittle stars have distinct spines along their arms, while their central body disc may appear more granular.
Sea cucumbers depart from the spiny appearance, as their ossicles are microscopic and scattered throughout their body wall. This gives their skin a leathery or soft texture. Crinoids, or sea lilies and feather stars, have less obvious external spines, with their ossicles often covered by soft tissues.
Other Unique Echinoderm Characteristics
Beyond their spiny skin, echinoderms are distinguished by several other biological features. A defining characteristic is the water vascular system, a complex network of fluid-filled canals and tube feet. This system begins with a madreporite, a sieve-like plate on the animal’s surface, allowing seawater to enter and exit.
From the madreporite, water flows into a stone canal, then into a ring canal, and finally into radial canals that extend into each arm or body section. Numerous tube feet, which are small, muscular appendages, branch off these radial canals. The water vascular system is important for various functions, including locomotion.
The system plays a role in feeding, allowing tube feet to manipulate food particles. Gas exchange and waste excretion also occur across the thin walls of the tube feet.
Adult echinoderms exhibit radial symmetry, specifically pentaradial symmetry, where body parts are arranged in fives around a central axis. This contrasts with their larval stages, which exhibit bilateral symmetry, a common characteristic.