groups include: above the eyes (supraorbital) on the cheeks (genal) where a moustache would be (mystacial) under the snout (mandibular). The mystacial whiskers can be roughly identified as macrovibrissae (long whiskers for feeling the space around the head) and microvibrissae (small, down-pointing whiskers for identifying objects). Not only are these two types hard to distinguish on an animal's face (see for example the image of a rat here), there are similarly weak distinctions in how they are used, though the distinction is nonetheless referred to ubiquitously in scientific literature and is considered useful in analysis. Many land mammals, including domestic cats, also have vibrissae on the underside of the leg just above the paws (called carpal vibrissae). Whilst these five major groups are often reported in studies of land mammals, several other groups have been reported more occasionally; for instance nasal, angular, and submental whiskers. Marine mammals can have substantially different arrangements of their vibrissae. For instance, whales and dolphins have lost their snout whiskers and gained vibrissae around their blowholes, whereas every single one of the body hairs of the Florida manatee may be a vibrissa (see image). Other marine mammals, like seals and sea-lions, have head vibrissae just like those on land mammals (see image), although these groups function quite differently. Vibrissal follicles have evolved other functions in dolphins, such as electroreception. Vibrissae The vibrissal hair is usually thicker and stiffer than other types of (pelagic) hair but, like other hairs, the shaft consists of an inert material (keratin) and contains no nerves. However, vibrissae are different from other hair structures because they grow from a special hair follicle incorporating a capsule of blood called a blood sinus which is heavily innervated by sensory nerves. Vibrissae are symmetrically arranged in groups on the face and supply the trigeminal nerve. The mystacial macrovibrissae are shared by a large group of land and marine mammals (see images), and it is this group that has received by far the most scientific study. The arrangement of these whiskers is not random: they form an ordered grid of arcs (columns) and rows, with shorter whiskers at the front and longer whiskers at the rear (see images). In the mouse, gerbil, hamster, rat, guinea pig, rabbit, and cat, each individual follicle is innervated by 100–200 primary afferent nerve cells. These cells serve an even larger number of mechanoreceptors of at least eight distinct types.