Invertebrates

Apart from insects, which have extensively colonized terrestrial environments, most invertebrates are aquatic, many of those being marine. Even some of the land dwellers start life as aquatic larvae. Invertebrates follow one of three types of general body plan. Some marine sponges are asymmetrical, lacking an ordered pattern to their structure. Cylindrical organisms, such as sea anemones, are radially symmetric—any cut through the center of the organism divides it in equal halves. Asymmetrical and radially symmetric animals tend to stay in one place, and their body plan helps them collect environmental stimuli from every direction. Animals that exhibit bilateral symmetry have right and left halves that are mirror images of each other. They are mobile and usually have a distinct head end. This area of cephalization, concentrated nerve and sensory tissues, is directed forward in their travels, giving them new information about where they are going.

Invertebrate Feeding

Among the protozoans, there are a variety of diets and feeding mechanisms. Organisms in the phylum Ciliophora have a cytostome, a cell mouth that can be found anteriorly, laterally, or ventrally, depending on species, on these single-celled creatures. Ciliates feed primarily on bacteria, algae, and other protozoans. Members of the phylum Amoebozoa have a similar diet to the ciliates but, in the absence of mouths, use pseudopodia to wrap around a food item, engulfing it. Euglena (phylum Euglenozoa), a flagellate protozoan commonly used in biology laboratories, is a self-feeder (autotroph). It contains chloroplasts and uses light energy to produce sugars, photosynthesized as in plants. It is interesting to note that some euglenoids can, and do, ingest solid food if they are exposed to darkness for too long. Another group of interesting flagellates is the hypermastigotes. Species such as Trichonympha campanula live in the guts of termites. T. campanula breaks down the high cellulose content present in the termite diet of wood products, something that the termite cannot do for itself. In return these protozoans keep some of the nutrients for themselves.

Sponges, phylum Porifera, do not have any organs, but they do possess specialized cell types. Choanocytes, also termed collar cells, line the inside of the sponge and capture small food particles present in the circulating water. The phylum Cnidaria is well known for quite another specialty. Organisms in this phylum, such as jellyfish, have specialized stinging cells called nematoblasts. One of the functions of cnidae within the nematoblasts is secretion of toxins used to paralyze and kill prey. Cnidarians are patient hunters, lying in wait until their next meal contacts a tentacle or two, triggering the toxic sting.

Tapeworms (phylum Platyhelminthes, class Cestoda) have no mouth or digestive tract. They are highly adapted to a parasitic way of life. Swimming in nutrients that their host, usually a vertebrate, is in the process of digesting, tapeworms absorb nutrients through their outer surface, and return nothing besides waste products to their host.

Many rotifers (phylum Rotifera) are omnivores, meaning they eat anything that will fit into their mouth. After being ingested, prey passing into the muscular pharynx encounters grinding, crushing jaws called trophi. Rotifers have a one-way digestive tract. Wastes pass out the anus rather than being expelled through the mouth, unlike many of the animals discussed thus far.

There is a wide range of feeding styles among the mollusks (phylum Mollusca). Some feed on plants; others feed on animals, and others feed on particles suspended in the water or mud that they themselves inhabit. There are also mollusks that are ectoparasites, living on the exterior of their host rather than inside it. An interesting molluscan feeding structure is the radula. The teeth on this tongue-like organ are replaced as they wear down or break.

The phylum Arthropoda is divided into two subphyla based on mouthpart structure. The Chelicerates, including horseshoe crabs, and arachnids (spiders, mites, ticks, and scorpions), have fang-like oral appendages used to grab and shred food items. Conversely, insects and crustaceans generally have mandibles, a pair of jaws for crushing food. This basic plan can be highly modified. The specialized coiled straw proboscis of a butterfly is such an example.

Starfish (phylum Echinodermata) prey on large invertebrates and small fish. These echinoderms have two stomachs and can protrude the cardiac stomach through the mouth and begin to digest prey externally. This is an especially useful maneuver when feeding on particularly large prey or when eating a clam through the small opening between its shells.

Respiration and Circulation

As single cells, protozoans have a high body surface to volume ratio. They inhabit wet, or at least moist, environments and can take care of gas exchange through simple diffusion. Even though flatworms, phylum Platyhelminthes, are multicellular, they also possess a large external surface area relative to their internal volume, so they, too, rely on simple diffusion across their body surface area for gas exchange. A highly branched gastrovascular cavity allows most cells to be in contact with the digestive system, which means that nutrients do not have to circulate to remote parts of the body. Metabolic wastes generally diffuse out across the body surface.

Sponges rely on their body cavity (the spongocoel) for circulation and gas exchange. Seawater with dissolved oxygen is pulled into the spongocoel through pores in the body wall called ostia. Flagella in specialized collar cells lining the spongocoel set up an internal current to provide circulation. Water and waste products then pass out of a larger opening, called an osculum.

Cnidarians also lack specialized respiratory structures. They rely on epidermal and gastrodermal surfaces for gas exchange. The gastrodermis lines the gastrovascular, main body cavity. As its name suggests, the gastrovascular cavity functions in both gastric capacity of digestion and the vascular role of circulation.

Most aquatic mollusks have comb-like gills that function in gas exchange and in filter feeding in some. Both aquatic and terrestrial snails in the subclass Pulmonata have highly vascularized structures that function as modified lungs. Mollusks typically possess a heart or similar pump that circulates fluid through an open circulatory system. In an open circulatory system, the conduits, or vessels, are limited, and most organs are bathed directly in the circulatory fluid. Cephalopods, such as squid and octopuses, are mollusks with closed circulatory systems. In a closed circulatory system, blood is contained in vessels. The squid has three hearts. The systemic heart receives oxygenated blood from the gills and sends it to the tissues. The two branchial hearts pump deoxygenated blood back to the gills.

Arthropods have open circulatory systems. The circulatory fluid, called hemolymph, enters the heart through holes, called ostia. It is then pumped through short arteries and into the body cavity. There are several adaptations for gas exchange among the arthropods. Spiders have book lungs consisting of multiple stacked plates, like pages in a book. Insects have trachea, tubes that connect to the outside through holes (spiracles) in the exoskeleton. The trachea transports air directly between body tissues and the environment. Rounding out the arthropods, aquatic crustaceans respire using filamentous gills.

The water vascular system (WVS) is unique to the phylum Echinodermata. This system of canals services thousands of tube feet, in the starfish, for example. These feet extend through the body wall and have many other functions in addition to locomotion. Due to circulating fluid within the WVS, tube feet also are involved in gas exchange, waste excretion, chemoreception, and food collection. Another echinoderm, the sea cucumber, possesses internal rather than external respiratory structures. These respiratory trees attach to the cloaca, the common collection area for the exit of digestive and reproductive systems from the body. Gases are exchanged via the pumping of seawater in and out of the cloaca.

Reproduction and Development

Reproduction among invertebrates is almost as varied as the animals themselves. Cnidarians and sponges are just two of the many phyla that reproduce asexually, offspring arising by breaking off from the parent organism. Groups such as rotifers and some arthropods alternate between asexual and sexual reproduction, depending upon environmental conditions. The term parthenogenesis is used to describe development of an egg in the absence of fertilization. Parthenogenesis tends to occur during stable, favorable conditions. Sexual reproduction, the mixing of genes from two parents through uniting of egg and sperm, produces individuals with new combinations of genes, which may adapt them for survival under stressful conditions.

There are variations in sexual reproduction as well. Some invertebrates are hermaphrodites. These animals, such as earthworms, possess both male and female reproductive systems. Some can fertilize themselves; others cannot. When hermaphrodites mate with other members of their species, each individual donates and receives sperm, resulting in twice as many offspring per mating. Gonochoristic species have separate sexes, an individual being either male or female. This is the case for most insects.

In marine invertebrates, fertilization may be external. This involves broadcasting sperm and eggs into the surrounding seawater and relying on the ocean currents to bring the two together. Internal fertilization is the rule for freshwater and terrestrial invertebrates. Sperm may be transferred directly into the female’s reproductive tract through a copulatory organ such as a penis. An indirect method of sperm transfer involves a package called a spermatophore. This chemical packet commonly provides nutrients for the female and her resultant offspring.

Aquatic larvae are commonly the dispersal stage in the life history of the particular invertebrate. Many times, the adults are sedentary or even sessile, anchoring themselves to the ocean floor or remaining in self-constructed burrows. The larvae, on the other hand, are planktonic, relying on the ocean currents for transportation. A similar scenario exists with spiders. The young spin silken parachutes and use the wind currents to disperse away from the web of the mother spider. However, for many terrestrial invertebrates, the young represent the feeding stage. Lepidopteran caterpillars eat voraciously, and some adults, such as the luna moth, do not eat at all, living strictly off food stores acquired as a larva. Especially with the winged insects, it is the adults who colonize new areas.

Principal Terms

Coelom: A true body cavity, lined by mesoderm

Colony: A cluster of genetically identical individuals formed asexually from a single individual

Cyst: A secreted covering that protects small invertebrates from environmental stress

Gonochoristic: Having separate sexes; an individual is either male or female

Hydrostatic Skeleton: A system in which fluid serves as the support by which muscles interact

Mesoderm: A middle layer of embryonic tissue between the ectoderm and endoderm

Bibliography

Barnes, Robert D. Invertebrate Zoology. 9th ed. Saunders College Publishing, 2008.

Campbell, Neil A., Jane B. Reece, and Lawrence G. Mitchell. Biology. 12th ed. Benjamin/Cummings, 2021.

Knutson, Roger M. Fearsome Fauna: A Field Guide to the Creatures That Live in You. W. H. Freeman, 1999.

Pechenik, Jan A. Biology of the Invertebrates. 7th ed. McGraw-Hill, 2022.

"Invertebrates." Smithsonian Ocean, ocean.si.edu/ocean-life/invertebrates. Accessed 25 June 2023.

Robertson, Matthew, ed. The Big Book of Bugs. Welcome Enterprises, 1999.