Sea Carnival

Corals and anemones, together with jellyfish, make up the phylum Cnidaria. Some characteristics they share are their bright colors, tentacles that secrete stinging substances, and a digestive system with a common opening for ingestion and excretion-the simplest digestive system in the animal kingdom. All of these organisms are quite simple. Corals generally form colonies, large groups of small polyps that stay practically immobile and feed on microorganisms brought to them by water currents. Sea anemones, on the other hand, are solitary and can trap prey despite their limited locomotion. •

^^ Corals are small polyps with ^^ tentacles that grow at their base throughout their life, generating a calcareous exoskeleton. This skeleton forms masses, or branches. Most corals grow in colonies; the skeletons form huge calcareous masses called reefs. Corals live mostly in warm, shallow ocean waters. Their reproduction can be both sexual and asexual, by division or by gemmation. They feed on plankton.

HARD CORALS

grow over the surface of the lime-bearing substrate.

SOFT CORALS

branch out; their skeleton is not lime-based but horn-like and flexible.

CORAL WALLS

Even though some coral walls live alone, most form colonies that can grow upward at up to 3 feet (1 m) every year.

THE MOST COMMON DEPTH AT WHICH CORALS GROW

Tentacles with stinging cells

Hard skeleton A mass that grows by the accumulation of dead polyps

Through here the animal ingests its food and excretes wastes.

Live tissue

Gastric cavity. Divided into several cavities in hydropolyps

Connecting tissue

Connects one polyp with another

Calcium carbonate

Beautiful but Deadly

^^ Beautiful for their shapes and colors that ^^ vary even within the same species, and dangerous for the poison they use to sting both victims and predators, sea anemones live in almost all marine latitudes, and at varying depths. Tropical marine anemones can measure up to 3 feet (1 m). They have a basal disc, which allows some species to attach to rocks, and others to slither, and still others to penetrate the seafloor. They trap live prey, even fish, with the many tentacles around their mouths.

ADAPTATION OF SHAPE

To avoid being swept away in the current, the sea anemone retracts on sensing a water flow.

Water flow

DISTENSION

1JMP

- Tentacles

- Column

Base

CONTRACTION

DISTENSION

EXTENSION

GASIROVASCULfllR

9,000

THE NUMBER OF CNIDARIAN SPECIES IN THE WORLD

The sea anemone reduces its size.

By means of the retractor muscle

TENTACLES

With stinging cells, to hunt and move

When the water is calm

CLOWNFISH

Inexplicably, the sea anemone's poison does not affect this species.

RHARYNM

SEA ANEMONE

Any vertical plane passing through its center divides it into two equal parts.

Aquatic

Echinoderms (phylum Echinodermata) are one of the best-known groups of marine invertebrates. Sea urchins and starfish, despite their apparent differences, are part of the same group and share characteristics such as five-radial symmetry. This phylum has an aquatic vascular system with many ambulacral grooves with tube feet, which it uses for locomotion, capturing prey, and breathing. In addition, it has an internal skeleton made of calcareous plates. These creatures lack a brain or eyes, so they use photoreceptors to sense movement. •

Defense System

^^ Characterized by complete five-sided symmetry, sea urchins' bodies are covered by several mobile spines that give them a dangerous appearance. The spines are spread evenly throughout the skeleton's surface and are used as a defense system.

Aquatic

Ambulacral Grooves

PYLORIC CONDUIT

Moves the water that ends at the pyloric cecum, which functions as a digestive gland

The muscles can make the tube feet move to either side, and the coordinated movement of all the feet in one direction causes the starfish to advance.

Ambulacral Grooves

SEA URCHIN

Astropyga radia ta

SPINES

There are two varieties of spines: larger primary spines and shorter secondary spines. They are usually cylindrical with a narrowed tip.

PYLORIC CONDUIT

Moves the water that ends at the pyloric cecum, which functions as a digestive gland

These structures are hollow cylinders with thick walls that straighten and move when a starfish injects water into certain vesicles in its body. The ambulacral grooves end in suckers that the animal uses to attach itself to objects, enabling it to move at surprising speed. These sensitive feet shrink if touched abruptly, hiding behind a rim of rigid spines that protect them from harm.

The muscles can make the tube feet move to either side, and the coordinated movement of all the feet in one direction causes the starfish to advance.

The area near the sucker secretes an adhesive substance that helps keep it attached to the surface. The lateral muscles in the groove contract, and the liquid returns to the sac for locomotion.

Stages of Movement

When the sac muscles contract, they force the liquid to pass to the ambulacral groove, which lengthens and makes contact with an adjacent surface, or substrate.

The ambulacral groove and tube feet allow the starfish to perform the movements it needs for locomotion. The feet are arranged in two parallel lines along the arm, and the feet at the other end have a sensory function, monitoring the substrate over which the creature moves.

26 THE SIMPLEST LIFE-FORMS

Worms are invertebrates with long, soft bodies and no legs. They are classified into three phyla. Flatworms are the simplest type; most are parasites, although some are free-living. Nematodes have a cylindrical body with a hard outer surface. Segmented worms are more complex; they include leeches, earthworms, and sea worms. Many species have an impact on plants, animals, and humans. •

WORM

Nematode enopHda

Movement Guided by Light

Flatworms have eyespots, or light-sensitive eyes, on the front end of their bodies. When exposed to excessive light, the eyes withdraw and remain immobile.

PROBOSCIS

Partly folded inward

Digestive System

In annelid worms, the digestive system extends in a straight line from the oral opening to the anus. It includes the mouth, muscular pharynx, esophagus, crop, gizzard, and intestine.

Hold the worm in place

Hearts

Clitellum

Mouth

Pharynx

Reproductive system

Intestine

LENGTH OFJHE LONGEST WORM: PL A CENTQNEMAf GIGANTISSIMUM I

EARTHWORM

Lumbricus terrestris

LOCOMOTION

Snake-like undulations along the dorsal-ventral plane

Setae are bristle-like structures.

LEMNISCI

Food storage new worms.

FOOD

Bacteria and organic wastes are formed in layers and are based on the presence of internal cavities. This annelid has three layers and one cavity, the coelom, which carries fluids through the body like a hydraulic skeleton.

DIGESTIVE CAVITY

AT LEAST

100,000 KNOWN WORM SPECIES

Retracts and remains hidden

TISSUE

Fibrous and elastic

Reproduction

Flatworms and annelids are usually hermaphrodites; nematodes usually have separate sexes. In some cases the worm splits into two, resulting in two

Pierce the wall of the host

Tissues

Was this article helpful?

0 0

Post a comment