AP Biology Ailin Zhang: Sem 2 Week 3 Starfish Description

Starfish is classified under the phylum Echinodermata.

Symmetry
A starfish typically possess a 5-rayed symmetry. Most starfish have a radical symmetry, but some has bilateral symmetry.

Germ cell layers
The body wall of echinoderms consists of three layers. The outer layer is called the epidermis, which is a single layer of cells which covers the entire animal including its various spines. The middle layer is much thicker compare to the epidermis, which is called the dermis. The dermis is composed of connective tissue and contains the exoskeleton. This exoskeleton takes three different forms: a set of closely joined plates with little individual movement that exist as a test or shell, a set of separarely ariculating small plates called ossicles, and a collection of widely separated microscopic ossicles lying in the dermis. The ossicles are always made from calcite. The exoskeleton of echinoderms grows continuously throughout the animals life. The exoskeleton supports the spines, warts, and tubercles that are often found on the echinoderm surface. These various protuberances are also generally made from calcite. The third layer is also a single layer of cells. The main difference is that these cells are ciliated. This layer enclosed the animal's coelom separating the animal's guts from its skin (called coelomic lining).

Coelom
Starfish has a true coelom body cavity.

Circulatory system
Echinodermata has a poorly defined open circulatory system and posses a water vascular system, which hydraulically operates the tube feet or feeding tentacles. Echinoderm contact with the external world through its water vascular system. The water vascular system of the echinoderms is unique in the living world and easily distinguishes them from all other phyla. The system takes slightly different forms in the different classes. In the Crinoidea, which are believed to be the most ancient of the echinoderms, the tube feet are branched and secrete mucous. In the Ophiuroidea the tube feet are simple and slender. In Asteroidea, Echinoidea and Holothuroidea they are thicker and end in suckers. The water vascular system starts with an opening to the external environment called a madreporite. From this a short straight canal called the 'stone canal' leads to the 'ring canal'. The ring canal is a ring as might be expected and it has five longitudinal canals branching off from it into each of the arms, or their morphological equivalents in Echinoidea and Holothuroidea.

Reproduction
Starfish reproduce sexually and gonochoristically. Fertilization is always external. The larvae are planktonic and biradial. Develop into pentaradial symmetry as adults.

Feeding
Most Echinodermata possesses a through gut with an anus. The gut is U-shaped in the Crinoidea with the mouth and anus being on the same surface. Other groups have straight-through gut with mouth and anus on opposite sides. They are filter feeders. Echinodermata feeds on fine particles in the water, detritus, or other animals living in marine environment.

Excretion
Without excretion organ. Use the water vascular system to excrete waste. Many uses their tube feet as organs for gaseous exchange.

Nervous system
The nervous system includes a circum-oesophageal ring. Echinoderms have sub-epidermal nerve net running all over their body. They also have a circum-oral nerve ring with 5 radial nerve cords, connecting sub-epidermal nerve net.

Crinoidea (Sea lilies, feather stars, and comatulids)