Mermaids

Morphology & Physiology

Mermaids are one of the few sapient aquatic species, distinguished by a Cytan upper body, trailing into a long, muscular tail below the hips. The tail is roughly ovoid in shape, slightly flatter at the front, and deeper at the back. It ranges in thickness from about twice the width of the Cytan waist, tapering off slowly, then sharply as it approaches the caudal fin (or tail tip). Depending on the individual, the tail may range from 1.5 to over 3 meters in length, and comprises the bulk of the mermaid's volume and mass. It is an extension of the spine, and highly flexible in the vertical direction, less so in the horizontal. A sinuous up-and-down motion is used to generate force, and therefore propulsion. This tail is coated in a dense layer of small scales, ranging in size from a little less than 1 centimeter to under 600 microns, depending on location and subspecies/individual. The overall result is that the tail is smooth and sleek, at least from the front. The tail also features two sets of fins - one at the tip, forming a large horizontal 'fluke', with a rough T, V, or delta shape, which serves to catch the water and provide the bulk of the tail's thrust. These 'caudal fins' are motile, comprised of a slab of dense muscle and cartilage, and can be adjusted to steer and or optimize for different conditions. A thin sheet of scale, supported by cartilage ribs, extends beyond the main body, providing additional traction. This feature varies in size based on subspecies/individual, and may be vestigial. The second set of limbs, the pelvic fins, are similar in basic structure, but drastically differ in form and function. Instead of forming a single connected structure, the pelvic fins are found independently on either side of the upper tail, a few decimeters below the hips. Smaller in size than the caudal fins, they serve to aid in steering at both high and low speeds, as well as covering the gill vents. Cytan morpology is typical, with the addendum that all adult mermaids are homologous females. The hands are not, as is sometimes believed, webbed, and the arms serve no function in high-speed movement, used solely for manipulation, or occasionally for additional maneuverability at low speeds.

Respiration

Mermaids are capable of breathing both water and air, and are highly resilient to many different levels of salinity, accomplished through a complex system occupying approximately half the chest, divided into two main parts: a single true 'forelung' and a pair of wholly internal gill pouches. In addition, the diaphragm is highly complex, able to move fluids both via traditional diaphragmic breathing and peristaltic compression.
Offset slightly left beneath the sternum, the forelung is primarily adapted to breathe air, a branch-lobed sack functioning like the lung of any terrestrial creature. While it can derive oxygen from water, it does so fairly inefficiently, and the main bronchi contract while breathing water, partially sealing the lung and diverting flow to the gills. The forelung is also used for respiration at low metabolic intensity, speech, and - through regulation of dissolved gases - helps in maintaining bodily buoyancy and pressure. However, when exerting significant effort underwater the forelung is insufficient.
The gill chambers are located on either side of the chest, and are considerably larger than the forelung. Unlike in ordinary fish, they are entirely internal, protected within the ribcage. Each chamber consists of a pouch wrapped in the peristaltic diaphragm and filled with folds and channels. Water is drawn into the body and passed into these channels, where numerous vascular filaments filter oxygen from the water, with a total surface area of roughly 600 square meters. The deoxygenated water than exits the bottom of the gill chamber where it downwards, exiting via a series of 'spiracles' just below the pelvic fins. To facilitate this, the peristaltic diaphragm is capable of 'pumping' water through the gills fashion, rapidly expelling deoxygenated water and drawing in more water.
The spiracles are normally loosely closed, covered by a muscular cartilage 'flap', which aids in preventing backflow. They are not, strictly, part of the gills, and are closer to a nostril than a fish's gill arch.

Circulation

Circulation through the body is conducted in a fairly typical fashion for a Cytan species, with a large heart on the right side of the upper torso (across from the forelung). Oxygenated

Genetic Ancestor(s)

• Cytan

Scientific Name

Ecchen Mas-Terlitannen Dorinen Ise-vela Cytan Vieson Ni-i-taadi

Average Height

250-300 cm

Average Weight

120-200 kg