Species

**Species** are unique lifeforms. They are generally divided into three classifications based on their level of sentience: sentient, semi-sentient, and non-sentient. There were over 5 million sentient species in the galaxy known to the Federation, and a few from other galaxies as well. Sentient species are sometimes erroneously referred to as **races**, which more accurately refers to groups or subspecies within a species.

## Overview

### Lifeforms

The five characteristics of lifeforms were organization (a distinct form and appearance), irritability (the ability to respond to external stimuli), metabolism (the ability to convert energy to stay alive), reproduction (the ability to create new lifeforms), and adaptation (the ability to change as their environments change).

The predominant form of sentient life in the galaxy was carbon-based lifeforms. However, other forms of sentient life did exist, such as silicon-based or energy-based lifeforms.  Across all species, the majority were oxygen breathers, although a number breathed other substances such as ammonia, methane, and helium. Despite these differences, most sentient species were easily classified into a number of distinct categories:

**Mammalian sentient species** are warm-blooded vertebrates, who nourish young through secretions from mammary glands. Most mammals were covered in hair, and many were humanoid. Many of the humanoid sentient species evolved from lower lifeforms resembling primates, or from creatures resembling felines or other types of mammal. Some of the most prominent species in the galaxy are either mammals or mammal-like, such as humans and Vulcans. Other notable forms include the Jiralhanae, San'Shyuum, Na'vi and the Kzinti.

**Reptilian sentient species** are cold-blooded vertebrates, typically covered in scales or bony plates. They typically evolved from non-sentient species resembling lizards, snakes and crocodiles. Most reptiles are land based, and lay eggs to reproduce. Reptilian sentient species were also very common in the galaxy, and notable examples included the Voth, Drell, Cardassians, and the Xindi reptilians. They also came in less humanoid forms, such as the Sangheili and the Gorn.

**Avian sentient species** are a subset of reptilian species, usually warm-blooded, feathered, and winged. They typically had the light bones and strong muscles needed to enable flight. However, many sentient avian species have lost the ability to fly. Examples of avian species included the Turians, Raloi, Aurelians, and the Burani. The Xindi avians, though extinct by the time of modern galactic history, were another example.

**Insectoid/arachnoid sentient species** are arthropods, with defined body segments; most commonly head, thorax and abdomen. They are distinguished by their similarities to insects or arachnids. Many have hard exoskeletons and wings, and can have any number of segmented legs. Species such as the Arachnids, Rachni, Keepers, Mantids, and Yanme'e are classic examples of insectoid sentient life. Other insectoid species evolved into more humanoid forms. The Collectors, the Amengi, and the Xindi insectoids are examples of this form of life.

**Hard invertebrates** have hard outer bodies with no internal skeleton. These often included crustaceans and some flying creatures. While this classification also included insectoid species, they were usually grouped separately. The Unggoy are an example of a hard invertebrate sentient species. Despite their invertebrate status, Unggoy young were suckled from organs resembling breasts.

**Soft invertebrates** are descended from soft-bodied species with no bones which were often similar to creatures such as octopi, slugs and worms. Sentient species of this origin can take many diverse forms, such as the Lekgolo, Bandersnatchi, and the Hanar.

**Aquatic sentient species** are those descended from marine life. These lifeforms could often fit into distinct categories, but are usually classified together. Aquatic lifeforms include those that can breathe water, air, or both. These include the Antedeans, Salarians,Alonis and the Xindi aquatics.

**Craniopod sentient species** are, despite these broad categories, lifeforms so truly alien that they do not fit easily into any of the common categories. These were categorized as **exotic sentient species**.

### Study of sentient species

Many institutions in the galaxy, belonging to the Federation and other organizations, focused on the study of sentient species. Those scientists and anthropologists that studied sentient species were known as sentientologists.