Animals (Metazoa)

Some General Characteristics

Animals are multicellular organisms with eukaryotic cells that don’t contain cell walls. They are generally motile, or at least have motile larvae; heterotrophic (with the exception of some species with autotrophic symbionts); diploid (except for a relatively small number of haploid and polyploid species); and develop from a blastula (except for sponges).

Life Cycle

Most animal species reproduce sexually and have a binary mating system — they produce cells of two different mating types (in this case called sexes because the cells are of unequal sizes). In animals the two mating types (sexes) are called male and female, and their respective cells that undergo sexual reproduction are called sperm and egg. The sperm and egg cells, known as gametes, each have one set of chromosomes (whereas the other cells in an animal's body have two sets of chromosomes). In most cases, sperm cells are much smaller than egg cells and they are motile (propelled by a flagellum) whereas egg cells are nonmotile. A sperm cell fuses with an egg cell to form a diploid cell called a zygote, which develops into an embryo (passing through the blastula stage among other embryonic stages) and eventually into an adult animal.

Depending on the species, an individual animal may produce just sperm cells, just egg cells, or both sperm and egg cells.

Explained in slightly more technical terms:

The cells in an animal's body typically have 2n=2x chromosomes, where 2n indicates two times the number of chromosomes found in a gamete (sperm or egg cell) and 2x indicates two sets of chromosomes. One line of cells in an animal's body (called the germ line) produces gametes, through a process known as meiosis. The gametes have n=x chromosomes (the n indicates the # of chromosomes found in the gamete and the x written with no multiplication factor indicates one chromosome set). The gametes of the two different types (sperm and egg) fuse to form a zygote, which is diploid (2x) and contains twice as many chromosomes as found in a gamete (2n). The zygote undergoes development, eventually growing into a mature, multicellular animal.

Groups of Animals

Body symmetry

The great majority (>99%) of animal species have bilateral body symmetry (Holló and Novak 2012) (at least during a portion of their life cycle) — perhaps a testament to the advantage conferred by this type of symmetry. In bilaterally symmetrical organisms, the left and right sides of the body are mirror images of each other (i.e., there is one line of symmetry that can be drawn through an organism so as to divide it into parts that are roughly mirror images of one another). This type of symmetry appears to facilitate directional movement and may make movement in general more efficient. Animals with bilateral body symmetry are believed to have evolved from a common ancestor. The phyla containing the bilaterally-symmetrical animals are often grouped together into a clade called Bilateria. The only animal phyla not belonging to the Bilateria are the so-called “lower metazoans”: the Porifera, Cnidaria, Ctenophora, and Placozoa. The Porifera and Placozoa tend to be asymmetrical (lack anterior-posterior and left-right symmetry), the Cnidaria generally have radial or biradial symmetry, and the Ctenophora are typically biradially symmetrical.

Phylum Chordata

Below, groups of animals belonging to the phylum Chordata (the chordates) are shown on the left, while all the other phyla of animals that may be found in our area are displayed on the right. Most of the large and conspicuous animals that we are familiar with belong to phylum Chordata. An important characteristic that distinguishes the chordates from other animals is the presence of a notochord: a flexible rod made of a cartilage-like material that forms dorsal to the gut. The notochord is, in most cases, a transient structure that is present only during embryonic development. Other embryonic features characteristic of the chordates include a hollow nerve chord (that forms dorsal to the notochord) and pharyngeal slits.

Phylum Chordata includes vertebrates (organisms which have backbones or vertebrae) and invertebrates (which lack backbones). All other phyla of animals contain only invertebrates. Invertebrates make up an estimated 97% or more of all animal species alive today (Center for Biological Diversity 2016, Margulis and Schwartz 1998).

Animals found in New England