Invertebrate Animals: Sponges to Insects
Porifera and Coelenterata
Porifera, such as sponges, and Coelenterata, such as jellyfish and polyps, were once thought to be plants because they are often found fixed to a rocky or sandy substrate.
Porifera (Sponges)
The body of a sponge is shaped like a bag and is perforated by numerous pores, which communicate through its interior canals. The main cavity communicates with the outside through a hole called the osculum.
Coelenterates
- Medusa: Umbrella-shaped with an opening at the bottom. They live floating in the water.
- Polyps: Tubular, sac-like, with the opening at the top. They live fixed at the bottom of the sea and can be grouped in colonies, such as corals or anemones.
Worms
Earthworms, tapeworms, and leeches are examples of worms. The term “worm” encompasses animals with bilateral symmetry and a soft body without a skeleton.
Platyhelminthes (Flatworms)
Their body is flat, thin, soft, segmented, and without organs of locomotion. They are hermaphrodites, meaning their body has male and female organs, and they can self-fertilize and produce thousands of eggs. Examples include aquatic planaria, which are found in wet environments, and tapeworms, which are parasites of both people and animals. They lack respiratory and digestive systems.
Annelids
Earthworms and leeches are annelids. They have a soft body divided into cylindrical rings. All rings are similar and have the same organs. This repetition of rings on its body is called metamerism. Most are aquatic. They can be hermaphrodites or have separate sexes.
Mollusks
Squid, mussels, oysters, and slugs are mollusks. Most are aquatic, living in marine or freshwater, although some, like the garden snail, live in very humid terrestrial environments.
Body Structure
- They have bilateral symmetry.
- Their body is soft and divided into three regions: head, visceral mass, and foot.
- The foot is muscular and used to swim, crawl, or dig, depending on the type of mollusk.
- The body is covered by a thin membrane called the mantle, which produces a shell that protects the animal. The shell can have two halves called valves.
Physiology
Aquatic mollusks breathe through gills, while terrestrial mollusks breathe through lungs. Most mollusks are hermaphroditic, but there are species that have separate sexes. Most are oviparous. After fertilization, a larva is born, which undergoes metamorphosis and turns into an adult. They are classified as bivalves, cephalopods, and gastropods.
Gastropods
Snails, slugs, and limpets are gastropods. They can be terrestrial, like the snail, or aquatic, like the limpet. They have a well-developed head with four tentacles; the two eyes are located on the upper tentacles. The foot is very muscular and used to slither. In the mouth, they have a sort of tongue with small teeth called a radula. Almost all have a coiled shell that protects the visceral mass. Only slugs lack a shell.
Bivalves
Mussels, clams, cockles, and razor clams get their name from their shell, which is formed by two valves. They breathe by filtering water, which also provides their food.
Cephalopods
Octopuses, nautiluses, squid, and cuttlefish are all marine cephalopods. They have a very developed head with large eyes and a foot that has become tentacles surrounding the mouth. Most octopuses lack a shell. Others, like cuttlefish, have a small internal shell. They breathe through gills and are carnivorous. They use their tentacles, which are equipped with suckers, to capture their prey.
Arthropods
Body Structure
Their body has bilateral symmetry and is divided into three regions: head, thorax, and abdomen. In some, the head and thorax are fused, forming the cephalothorax. The head contains the antennae, eyes, and mouthparts. The sense organs are very developed. They can have simple eyes, called ocelli, or compound eyes. They possess articulated appendages such as legs, wings, mouthparts, and antennae. The number of wings varies from one group to another. The body is covered by an external skeleton, or exoskeleton, made of a substance called chitin, which protects them from predators and desiccation and prevents the body from drying out due to the sun’s rays.
Physiology
They have a varied diet, which is related to their way of life. There are carnivorous, herbivorous, and scavenger arthropods. Most have separate sexes, with distinct males and females. They are oviparous, and fertilization is usually internal. In some, a larva hatches from the egg and undergoes metamorphosis to become an adult. They are classified as insects, arachnids, crustaceans, and myriapods.
Insects
Butterflies, beetles, grasshoppers, and flies are insects. They are the most numerous group of arthropods. The head typically has a pair of compound eyes, two or three ocelli, and a pair of antennae with olfactory and tactile functions. The structure of the mouth is very variable and depends on the type of diet.
The thorax has three pairs of jointed legs and usually one or two pairs of wings. The shape of the legs depends on the insect’s lifestyle. The abdomen is segmented and does not have appendages. They breathe through tubes called tracheae. They have separate sexes, internal fertilization, and are oviparous. A larva hatches from the egg and undergoes metamorphosis.
Metamorphosis of the Monarch Butterfly
Once the female lays the egg, a caterpillar larva emerges. After a period of growth, it goes through a maturation or pupal stage, in which it is wrapped in a cocoon and undergoes a series of changes that transform it into a butterfly.