Lower Animals, Invertebrates

Kingdom Animalia includes about 35 phyla. When many people think of animals, they only think of mammals, yet Class Mammalia is only one class in Subphylum Vertebrata, which is only one subphylum within Phylum Chordata. We tend to lump all the other phyla and other taxa within Vertebrata into a catch-all pseudo-group, the “invertebrates,” but this is about 95% of all animals! There are more species of insects than all other plants and animals together (The more conservative estimates are that there are over one million species of just insects.). Mammals are only a minuscule piece of the picture.

Animals live practically everywhere on this Earth. Insects, alone, inhabit nearly all possible environments on Earth with the exception of deep in the ocean, yet their close relatives, the crustaceans have representatives that live there. Many other animals in many phyla live in the ocean and in just about every terrestrial habitat.

In general, animal reproduction includes flagellated sperm and a larger egg which join in fertilization to form a zygote. This grows by mitosis to form a blastula (blasto = bud, sprout), an embryonic stage that resembles a hollow ball, then on to other embryonic stages. From this point, if the young resemble the adults, it is said that the embryo grows into young, which grow into adults. If the young are very different from the adults, it is said that the embryo grows into a larva (larva = ghost, specter) which grows and metamorphoses into an adult. Caterpillars and tadpoles are larvae, which have different food, habitat, and appearance than the corresponding adults. A larva undergoes metamorphosis (meta = between, with, change; morpho = form; -sis = the act of) to become an adult.

Before discussing the various animal phyla, it is useful to introduce some of the terminology which will be used to describe these animals. The back or top side of an animal is its dorsal (dorso = back) side, and its belly or bottom side is its ventral (vent(er) = underside, belly) side. The head or front end is called the anterior (ante = before) end, and the tail or back end is the posterior (post = behind, after). Animals with radial symmetry (radia = spoke, radius; sym = with, together, metr, -metry = measure, measurement) do have distinct top and bottom sides, but have no distinct left and right. Starfish, jellyfish, and sea anemones are examples of animals with radial symmetry. Animals with bilateral symmetry (bi = two; later = side) do have distinct left and right sides, and most animals with which we are familiar, such as earthworms, ladybugs, and dogs, have bilateral symmetry.

The body of an animal is made up of several layers of tissue. Ectoderm (ecto = out, outer, outside; derm = skin) is the outer layer of tissue. The epidermis (epi = upon, over), or skin, other outer layers, and the nervous system in vertebrates (not in all animals) are formed from ectoderm tissue. Mesoderm (meso = middle) is the middle layers of tissue. Mesoderm forms the muscles and most other internal organs. Endoderm (endo = within, inner) forms the inner layers, including the lining of the digestive tract in all animals and the liver and lungs in vertebrates. Often animals have a space in their bodies between several of these layers. Animal groups, like flatworms, with no such space are referred to as acoelomates. Animals, such as roundworms, which have a space between the mesoderm and the endoderm are called pseudocoelomates, and the space is called a pseudocoelom. Animals, including earthworms, insects, and humans, which have the space in between several of the mesoderm layers are called coelomates, and the space is called a coelom.

Animal Phyla

Phylum Porifera (pori = pore; fer = bear, carry)

These are the sponges. As adults, they are sessile (attached to one place), but have free-swimming larvae. Sponges are hermaphroditic, that is, they have both sexes in one body. This term is derived from the myth in which Hermes (Mercury) and Aphrodite (Venus) got together and had a son, who they named Hermaphroditus, who while swimming one day, somehow became united in one body with a water nymph. Sponges have no symmetry.

Phylum Cnidaria (cnida = a nettle), formerly called Coelenterata

This includes animals like jellyfish (Class Scyphozoa), coral and sea anemones (Class Anthozoa [anthe = flower; zoa = animal]), all of which are marine, plus Hydra, a freshwater genus (Class Hydrozoa). These animals have radial symmetry. They have a digestive cavity with one opening, which thus is called a gastrovascular cavity (gastro = stomach; vascul = a little vessel). The one opening serves as both a mouth to ingest food and for expulsion of any wastes. The bodies of cnidarians are described in one of two ways, depending on whether the opening of the gastrovascular cavity is ventral or dorsal. Animals like Hydra and sea anemones, which are sessile and have the opening and surrounding tentacles at the top of their bodies have a body shape called a polyp (polyp = many footed). Jellyfish, which float and have the opening and surrounding tentacles at the bottom, have a body form called a medusa (medusa = a jellyfish). Cnidarian tentacles serve to capture prey. These tentacles bear stinging cells, called cnidocytes, to subdue their prey, and some jellyfish cnidocytes irritating or toxic to humans. Cnidarians have both sexual and asexual forms of reproduction. Hydra, for example, will periodically develop lumps on the sides of their bodies that are either testes or ovaries. These make and release sperm or eggs, which then join to form a zygote, which eventually grows into a new hydra. Hydra also reproduce asexually by a process known as budding in which a new, small Hydra begins to grow from the side of a large Hydra, eventually pinching off when it is fully formed.

Phylum Platyhelminthes (platy = broad, flat; helminth = a worm)

These are the flatworms because their bodies are flat and ribbonlike. They have bilateral symmetry, but are acoelomates. They have a gastrovascular cavity with one opening. Most have no respiratory structures and “breathe” through their skin.
The free-living (water-dwelling) ones include planaria (Class Turbellaria). Planaria have eyespots to detect light. Some interesting experiments have been done to study their learning ability. They are hermaphroditic and reproduce sexually. They are also capable of asexual reproduction, and many experiments have been done to study the ability of cut planarians to regenerate new planaria or parts of planaria.
Parasitic flatworms include flukes (Class Trematoda), which have complex life cycles needing specific hosts. Genus Schistosoma is a parasite whose life cycle involves both humans and snails, hence the caution not to wade in water in tropical areas where this fluke occurs.
Tapeworms are another parasitic type of flatworm (Class Cestoda). Their first segment is a “head” with suckers and hooks to attach within their host’s intestines. Their bodies are made of a number of subsequent segments, each of which is pretty much on its own and separate from the rest. These segments don’t need a digestive system because they absorb food from the host’s digestive tract and don’t need a respiratory system because they absorb dissolved air (O₂), again, from the fluids in the host’s digestive tract. Thus, these segments are mostly reproductive system (imagine each segment as a very prolific female!). The segments break off and are passed out with the host’s feces. Other animals who accidentally ingest some of the infected feces (cattle eating grass near where other cattle have defecated) can acquire the parasite. Tapeworms can grow very large (long) and absorb so much food from their host’s digestive tract that they cause nutritional deficiencies.

Phylum Nematoda (nema, nemato = a thread)

These are called roundworms because their bodies are cylindrical. Some are free-living in aquatic habitats (including damp soil), while others are parasitic. They have a digestive tract with openings at both ends: a mouth and an anus, thus it is said that they have a tube-within-a-tube body plan. They are bilaterally symmetrical and do have a pseudocoelom between their mesoderm and endoderm layers. All of their muscles are oriented longitudinally (along the worm), only allowing for thrashing motions. They have sexual reproduction with definite males and females.
Trichinella (trichin = a hair; -ella = small; -osis = diseased condition or state) is the parasitic genus whose members cause trichinosis. These parasites live in their host’s muscles, and are often found in infected pork. Thorough cooking at high temperatures kills the parasites, but microwaving often does not get the center of a piece of meat hot enough for a long enough time to be effective. Because of this, while it is OK to reheat pork in a microwave, it should never be cooked that way for the first time. If a person ingests living Trichinella in undercooked meat, in the person’s intestines, the parasites will develop from larvae into adults and reproduce. Mated females will bore into the person’s intestinal muscles, where they will produce a new generation of larvae. These new larvae, in turn, either travel through the person’s lymph system or burrow through his/her body tissue to reach his/her muscles and other tissue where they will live.
Parasitic roundworms also include pinworm and hookworm. School children frequently contract pinworm from doorknobs, etc. at school because infected classmates don’t wash their hands after wiping feces from their anal areas. Pinworms live in the rectum and lay their eggs around the anus, thus one main sign of a pinworm infection is when a child frequently scratches his/her anal area. A classic test for pinworm infection is to touch a piece of scotch tape to the child’s anal area, followed by microscopic examination of the tape to check for the presence of pinworm eggs.
Many roundworms are free-living, but need to live in an aquatic environment of some sort, which can include damp soil. Many soil nematodes are beneficial and are important decomposers, but there are some which can be found in garden soil that damage desirable plants.

Phylum Mollusca (mollusc = soft)

There are several classes of mollusks: Class Polyplacophora (poly = many; plakos = flat plate, tablet; phora = bear, carry) which includes the chitons, Class Gastropoda (gastro = stomach; poda = foot) which includes snails and slugs, Class Bivalvia or Pelecypodia (bi = two) which includes oysters and clams, and Class Cephalopoda (cephalo = head) which includes octopus (octa = eight), squid, and chambered nautilus. Mollusks are soft-bodied and have a true coelom. Many have shells made primarily of calcium carbonate: chitons have a shell made of eight plates, snails have one, spiraled shell, clams have a shell composed of two, hinged halves, squid have small internal “shells,” and the chambered nautilus has a chambered shell (spiraling in living species but straight in many fossil forms). Snails and slugs have a radula (radul = a scraper), a rasping organ in their mouths to “scrape” bits of food into their mouth. Most bivalves use their gills to filter small food particles, which are then directed toward their mouths. Cephalopods have beaklike jaws for biting and crushing their prey, and unlike members of the other classes, tend to be fast-moving carnivores. Many mollusks have good eyes (snail, octopus, squid). Octopuses (octopi?) are famous for their well-developed nervous systems and brains (necessary to process the information from their sophisticated eyes), and they are known to be very intelligent. Most mollusks have males and females, while garden snails are hermaphroditic.

Phylum Annelida (annel = a little ring, a ring)

Annelid classes include Class Oligochaeta (oligo = few, scant; chaeta, setum = bristle), earthworms with few setae; Class Polychaeta (poly = many), marine worms with projections from each segment (parapods — para = beside, near; poda = foot) which serve as gills and bear many setae; and Class Hirudinea, leeches. Annelids are segmented, both inside and out. They have tube-within-a-tube body plan. Earthworms have a closed circulatory system with five pairs of “hearts.” Annelids have a ventral, solid nervous system which arises from mesoderm tissue (Compare this with the vertebrate nervous system which is dorsal, hollow, and of ectodermal origin.). Many of the internal organs, for example, the paired nephridia (nephri = kidney) which serve an excretory function analogous to our kidneys, are repeated in each segment. Earthworms exchange air (“breathe”) through their moist skin, thus if a worm dries out, besides the dangers of dehydration, it can’t get air so it dies. However, earthworms also cannot live underwater, and if the soil in which they are living becomes totally filled with water during a heavy rain, they will all come to the surface so they don’t drown (and end up on the sidewalk where they subsequently dry out after the rain stops). Earthworms are hermaphroditic and a pair of worms fertilize each other. Annelids have both longitudinal and circular muscles, so better control over their movement. (photos of earthworm anatomy)
Many leeches suck blood. Some have sharp jaws to slit their host’s skin, while others secrete an enzyme to digest a hole in the victim’s skin. It is to the leech’s advantage if the would-be victim remains unaware of the leech’s presence, thus most leeches secrete some kind of anesthetic (an = not, without; aesthet = sensitive, perception) so the host does not feel their attack. To prevent the blood from clotting before they ingest it, leeches also secrete an anticoagulant (anti = against, opposite; co- = with, together; ageve = to move, put in motion; coagulum = rennet). For years, people thought disease was caused by too much blood, so the Medicinal Leech Hirudo medicinalis was used to suck some out. Typically a medicinal leech, once attached, may suck for a couple hours, perhaps ingesting between one to two ounces of blood, many times its own body weight. While we now have different theories as to the cause of diseases, medicinal leeches still have uses in modern medicine! When someone severs a finger which must be surgically reattached, since all the capillaries are dysfunctional, blood flowing into the finger via the reconstructed arteries has nowhere to go but into the tissue. This leads to problems with edema in the area. Also, because of this poor circulation, the reattached tissues cannot get the air and nutrients they need to heal properly. It has been discovered that letting a leech suck on the end of the reattached finger will a) help reduce edema, and b) create a sort-of blood flow that will allow nutrients and air to get to the reattached tissues so they heal better and more quickly. Additionally, the anticoagulant secreted by the Medicinal Leech is very powerful, and a leech bite (in a person whose blood usually clots normally) can take a couple days to stop bleeding and form a scab. Research is being done on using this powerful anticoagulant to help heart attack and stroke victims whose problems are caused by blood clots.
Marine worms have a pair of gill-like structures called parapods projecting from the sides of each segment. It is believed that the early annelid ancestors of insects (and other arthropods) may have looked like this (and the parapods later became modified into true legs).

Phylum Onychophora (onycho = a claw, nail; phora, fer = to bear, carry)

Many biologists feel that, evolutionarily, this phylum is midway between Annelida and Arthropoda. The phylum includes genera Peripatus (peri = around, -patus = a walk, path) and Macroperipatus (macro = large, long), called the “walking worms.” One prevalent theory is that Onychophora is thought to have evolved from a marine worm-type ancestor, with the parapods evolving into unsegmented legs. This theory regards onychophorans as a sort-of “missing link” on the way to the evolution of insects (Other biologists believe that arthropods arose directly from annelids through some jointed-legged ancestor.). Onychophorans have unjointed walking legs with claws at the tips, and antennae and jaws which both are modified appendages like arthropods. Onychophorans are predatory and shoot sticky, mucus/saliva to tangle their prey. They have males and females and are live-bearing. Their excretory system consists of segmented, paired nephridia like annelids. They also share the presence and structure of their cuticle, their ciliated reproductive tract, and their musculature with Phylum Annelida. However, their respiratory system of tracheal tubes directly to the various organs, their open circulatory system, and their chitinous exoskeleton which must periodically be molted are shared with Phylum Arthropoda.

Phylum Arthropoda (arthro = joint; poda = foot)

This phylum fits in here, but we’ll come back to it later because it needs a longer discussion. It is thought that next step in arthropod/insect evolution was an early arthropod looked sort of like centipedes with one pair of appendages per segment (although without certain other specializations seen in modern centipedes).

Phylum Echinodermata (echino = hedgehog, sea urchin, spiny; derm = skin)

This includes starfish and sea stars in Class Asteroidea (aster = star), brittle stars in Class Ophiuroidea, sea urchins and sand dollars in Class Echinoidea, sea lilies or crinoids in Class Crinoidea, and sea cucumbers in Class Holothuroidea (holo = whole). Echinoderms are more closely related to chordates than any of the other invertebrate phyla. Their larvae have bilateral symmetry, but metamorphose to adults that have secondarily radial symmetry (have lost bilateral symmetry, rather than never had it). They have endoskeletons (endo = within, inner) with thin skins over them (vaguely reminiscent of our endoskeleton with skin on the outside). Echinoderms have an unique hydraulic system consisting of external, suction-cup-like tube feet connected by internal “plumbing.” By pumping fluid from one part of the hydraulic system to another, echinoderms can create a strong suction in the tube feet, aiding in movement and feeding. They have sexual reproduction with males and females. Much to the chagrin of early oyster fishermen, starfish are also capable of asexual reproduction by regeneration. Starfish prey on oysters, thus the oyster fishermen would cut up starfish to try to kill them, only to find the starfish population in their oyster beds had increased because one arm can regenerate a whole new starfish. Starfish have an unique way of feeding: they use hydraulic pressure in their tube feet to pull open a clam or oyster, and since this doesn’t involve the use of any muscles, the starfish doesn’t “get tired” pulling on the clam shell. The clam, however, must use its muscles to hold itself shut, and muscles can’t work forever without getting fatigued, so eventually the clam’s muscles will get tired and relax. When the clam tires and opens up, the starfish inverts its stomach into the clam and secreted digestive enzymes to digest the clam then and there.

Phylum Chordata

We will come back to this after we discuss the Arthropods.