Phylum Echinodermata and Transitional Invertebrates Phylum Echinodermata Cambrian to recent

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Phylum Echinodermata
Transitional Invertebrates

Phylum Echinodermata Cambrian to recent

  • All marine, bottom dwellers, (~6000 species). Many fossil forms.

  • Name means “spiny skin”

  • Exhibit Pentamerous radial symmetry (5-part)

bilaterally symmetric plankton


  • Only major invertebrate phylum that is a Deuterostome:

  • All higher animal forms(including chordates) are deuterostomes. Echinoderms considered to be more highly developed; (also protein structure is more similar to chordates)

  • Unique water vascular system provides a hydraulic system of tube feet: a series of suction disks used for locomotion; and finding and capturing prey.

  • Endoskeleton of calcareous spines or plates; covered by the epidermis.

  • Protects and supports internal tissues

  • Provides place for muscle attachment

  • Simple nervous system; un-centralized (no head)

  • Dioecious with external fertilization

  • There are 5 classes:

Class Asteroidea “Sea Stars”

  • Sizes ranges 1cm to 1 yard in diameter; many colors!

  • Most abundant in tropical areas

  • Thecais main body region

  • Anus is located on the Aboral (upper) surface; Madreporite- circular disk filter for water coming into water vascular system

  • Mouth located in center of oral surface; with ambulacral grooves that radiate out onto arms.


Class Ophiuroidea

  • Brittle Star, Basket star

  • long thin arms; clear demarcation from central disk. Break off easily

  • move by wipe like lashes or arms; are very mobile

Class Echinoidea

  • Sea urchins, sand dollars, sea biscuits

  • No arms; body enclosed in fused plates

  • Many movable spines

  • In sea urchins, the tube feet extend out through holes in the ambulacral plate. The spines are located on the interambulacral plate.

Class Holothuroidea- “Sea Cucumber”

  • Elongated body; spines small; soft body

  • These are detritus feeders

  • Can eviscerate (disembowel) themselves if threatened.

Class Crinoidea

  • These are the stalked Echinoderms

  • Most primitive Echinoderms

  • Filter feeders with feathery arms

Transitional Invertebrates

  • Also called Protochordates

  • Groups of animals that show some similar features to vertebrates

  • All deuterostomes (in larva the blastopore becomes the anus).

Phylum Chaetognatha “arrow worms”

  • Holoplankton (permanent plankton); have long narrow transparent body; hard to see (1-3cm); looks like glass

  • Name means “bristle-jaw”- stiff hairs around mouth; voracious plankton feeders

  • Very common in plankton samples.

Phylum Chordata


  • Have a notocord (a strong, flexible, rod-like structure) sometime during life cycle

  • Have a hollow dorsal nerve cord (above notocord)

  • Have gill slits (paired openings in wall of pharynx) sometime during their development

* This group includes all vertebrates

Subphylum- Urochordata

  • Called “tunicates” or “sea squirts”

  • Adult forms are simple, vase-like, usually transparent; sessile, benthic filter feeders (all marine)

  • Water is strained through gill chamber by ciliary action; also used in respiration

  • Begin development as a free swimming larva—tunicate larva

  • Chordate characteristics apparent only in the larva

  • Neoteny- condition in nature where the adult form is lost and the immature form becomes reproductive

  • Retention of juvenile characteristics in the adult form

Ex: 1) Tadpole---frog (adult form)

2) Salamander- immature becomes reproductive; adult form is lost

Subphylum- Cephalochordata

  • Means “chordate with head” (cephalization); feelers for sensory

  • Retains chordate characteristics throughout life (neotony); apparently arose from sessile ancestors

  • Called “lancelets” due to shape; have definite gill slits that function in filter feeding and respiration

  • Have strong striated muscles (for swimming) and small fins

  • Believed to “reflect” possible ancestor type of vertebrates

  • Earliest chordates date back to Cambrian Period called (graptolites)

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