Echinodermata with larval stages of the classes
- 1. ## *Phylum Echinodermata – The Spiny-Skinned Animals* ### *Overview* * Echinoderms are exclusively *marine invertebrates*. * Over *6,000 living species* are found at all ocean depths. * They *first appeared in the Cambrian period* (~538 million years ago). * Though adults appear *radially symmetrical, echinoderms evolved from **bilaterally symmetrical ancestors*. --- ## *Unique Characteristics of Echinoderms* 1. *Pentaradial symmetry* (five-part body symmetry in adults) * Larvae are *bilaterally symmetrical, adults are **radially symmetrical*. 2. *Endoskeleton* * Made of *calcareous plates or ossicles*. * Formed by *sclerocytes*, a type of dermal cell. * Ossicles contain *calcite microcrystals* arranged in a sponge-like lattice called a *stereom*. * Held together by *catch collagen* – allows flexibility and stiffness. 3. *Water Vascular System (WVS)*
- 2. * A *hydraulic system* derived from the coelom. * Powers *tube feet* used for movement, feeding, gas exchange, and excretion. * Sensitive to pressure changes. 4. *Pedicellariae* * Small pincer-like structures used for *cleaning, **protection, and sometimes **venom injection*. 5. *Dermal Branchiae (Papulae)* * Skin gills used for *respiration and excretion*. --- ## *The Five Main Classes of Echinodermata* ### 1. *Class Asteroidea – Sea Stars / Starfish* * *Body*: Central disc + 5+ tapering arms. * *Ambulacral grooves* open and visible; contain *tube feet with suckers*. * *Madreporite*: Located on aboral surface – filters water into WVS. * *Feeding*: Mostly predators; can evert stomach to digest prey externally. * *Reproduction*: * *Sexual: Separate sexes, external fertilization, **bipinnaria larvae*. * *Asexual*: Can regenerate arms or entire body from a portion of the central disc.
- 3. * *Ecological Role: Keystone species, e.g., *Crown-of-thorns controls coral populations. --- ### 2. *Class Ophiuroidea – Brittle Stars & Basket Stars* * *Body*: Small central disc, long slender articulated arms. * *Ambulacral grooves*: Closed. * *Tube feet*: Lack suckers, not used for locomotion. * *Feeding*: * Suspension or deposit feeders. * No anus; waste expelled through mouth. * *Respiration: Through **bursae* (sac-like structures). * *Reproduction*: * Mostly separate sexes; some hermaphrodites. * Gametes released into water via *bursal slits*. * *Larva*: Ophiopluteus. --- ### 3. *Class Echinoidea – Sea Urchins & Sand Dollars* * *Skeleton: Fused ossicles form a rigid **test* (shell). * *Spines*: Movable, used for protection and movement. * *Tube feet*: In five rows; have suckers.
- 4. * *Feeding*: * Use *Aristotle’s lantern* – complex jaw with 5 teeth. * Mostly herbivores; some are detritivores. * *Reproduction: External fertilization; **echinopluteus larvae*. --- ### 4. *Class Holothuroidea – Sea Cucumbers* * *Body*: Elongated along oral-aboral axis; appear bilaterally symmetrical. * *Skeleton*: Reduced ossicles; soft leathery skin. * *Tube feet*: Well developed in three longitudinal bands. * *Respiration*: * *Respiratory tree* – unique structure inside cloaca. * *Cuvierian tubules* for defense – sticky and sometimes toxic. * *Feeding*: * Modified *tentacles* trap food. * Deposit or suspension feeders. * *Reproduction*: * Single gonad, separate sexes. * Larva called *auricularia*. ---
- 5. ### 5. *Class Crinoidea – Feather Stars & Sea Lilies* * *Body: Flower-like with arms bearing **pinnules*. * *Sea lilies*: Sessile, stalked forms (deep sea). * *Feather stars: Free-moving, crawl using **cirri*. * *Feeding*: * Use *mucous-covered tube feet* to trap food particles. * Cilia move food to the mouth. * *Reproduction*: * Separate sexes. * Larva called *doliolaria*. --- ## Deuterostome Development (Compared with Protostomes) | Feature | Deuterostomes | Protostomes | | --------------------- | -------------------------- | ------------------------------- | | Mouth formation | From second opening | From first opening (blastopore) | | Cleavage pattern | Radial and indeterminate | Spiral and determinate | | Coelom formation | Enterocoely (outpocketing) | Schizocoely (splitting) | | Embryonic development | Regulative | Mosaic |