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Marine Invertebrates

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The Echinoderms lack a head and have five-point radial symmetry. These fascinating animals live only in marine environments. They have an endoskeleton made out of calcareous plates, which is often protected by spines. The plates that make up the endoskeleton often support the spines and enclose the coelom, an anatomical feature used for movement, respiration, collecting food, and as a sensory mechanism. The coelom also houses the reproductive organs and alimentary canal.

Sea star, Lembei Strait, Suluwesi, Indonesia 2008Echinoderms can be found in all oceans in all zones with approximately 6,000 described species.

The two main subphylums in phylum Echinodermata are Eleutherozoa and Pelmatozoa.

Subphylum Eleutherozoa conatins the superclasses Asterozoa and Cryptosyringida.

Superclass Asterozoa contains the sea stars/starfishes in Class Asteroidea and the extinct Class Somasteroidea.

Sea star, Lembei Strait, Suluwesi, Indonesia 2006Superclass Cryptosyringida contains Class Echinoidea (heart urchins, sand dollars, and sea urchins), Class Holothuroidea (sea cucumbers), and Class Ophiuroidea (basket stars, brittlestars, and snake stars).

Subphylum Pelmatozoa contains the Class Crinoidea (feather stars and sea lillies).

Mature echinoderms have five points that face outward from the center of the body with a mouth underneath and the anus on top. There are exceptions to this plan however; some echinoderms lack an anus and others, like the crinoids, have both the mouth and the anus on the same side of the body. Scientists refer to the side of the body with the mouth as the oral side and the side with the anus as the aboral side. Crinoids, ophiuroids, and holothuroids have tube feet to help collect food particles floating towards their body. Other types of echinoderms like asteroids are carnivorous and will surround or throw their stomach over their prey. Some echinoids even have teeth used to chew and dismantle plants and small animals.

Most echinoderms reproduce sexually producing larvae that feed on phytoplankton until they reach maturity. Some species of echinoderms develop their offspring in embryonic sacs located on the outside of their bodies.

Sea urchin, Lembei Strait, Suluwesi, Indonesia 2005Echinoderms have fascinating water-vascular systems that likely originated from some sort of respiratory system that evolved to include food gathering and movement. They accomplish these tasks through the use of their numerous hollow tube feet that resemble tentacles. There are two rows of tube feet on the outside of the body that fill with seawater so that when the animal expands or contracts, water is drawn into the feet. Once filled, the feet extend outward allowing the animal to walk. Suckers located at the tips of the tube feet are often used to grab prey or to hold onto solid objects when the echinoderm wants to remain attached to something.

Feather star, Lembei Strait, Suluwesi, Indonesia 2006The most familiar echinoderm known to humans is probably the sea star, categorized in the superclasses Asterozoa and Cryptosyringida. There are two classes of sea stars which include Asteroidea and Ophiuroidea. True sea stars and sun stars in are in Class Asteroidea while brittle stars and basket stars are in Class Ophiuroidea.

Echinoderms in the class Asteroidea have arms that are smoothly connected to the body; echinoderms in Ophiuroidea have arms that shoot out from a disk-like center. Both are able to regenerate their limbs when one is broken off. In some cases, a lost limb can generate a whole new sea star. The small bumps on top of the sea star are referred to as dermal branchiae and are used to absorb oxygen from the water for respiration. Pedicellaria are small appendages used to keep foreign bodies off of the sea star. The madreporite is a hard opening on the aboral side of the sea star used to regulate and filter sea water.

Sea stars also have an eye-like structure at the end of each arm, called the eyespot, used to detect light.


Hemichordates are a relatively small phylum. These creatures are extremely important to the study of the evolution of vertebrates. They are characterized by a body divided into three main areas: the preoral lobe, the collar, and the trunk. Hemichordates are partial chordates and are closely related to the first chordates. According to DNA analysis, hemichordates are closely related to echinoderms, which is also apparent during observations of hemichordate and echinoderm larval stages. Hemichordates have gill slits, a structure that resembles a notochord but is called the stomochord, a dorsal nerve cord, and a reduced ventral nerve cord.

There are three classes of hemichordates which include Enteropneusta, Pterobranchia, and Graptolithina. The most well-known class is the Enteropneusta or “acorn worms”. Acorn worms have gill slits, burrow into the sediment, and likely feed on dirt and detritus. They can reach up to 2.5 m or 8 ft in length but most are actually quite small. In the Pterobranchia class, there are only a few species notably different from the acorn worms. Pterobranchs live in colonies connected by stem-like stolons. Each tiny individual is referred to as a zooid and has one gill slit. The Graptolithina are most well-known in the fossil record showing up in the Ordovician and Silurian times.


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