The colours of these animals are dark grey, olive green, or brown to brick red on the dor sal surface and light, often almost white on the under surface. Different species present slightly dif ferent colour-patterns, but in more than one locality the same species ranges from black through olive green to reddish brown. Both head- and tail-ends taper and there is no distinct head. The anterior end bears two characteristic antennae, very mobile and extensible. In fact the whole animal is remarkably extensible and there is a great difference in length between a living specimen in motion (especially if this is rapid) and one in spirit.
Slightly posterior and ventral to the antennae are two small blunt oral papillae, and between these the buccal cavity is situated (fig. 2). An anal aperture is found at the extreme posterior end of the animal and further forward on the ventral surface between the last pair of legs is the repro ductive aperture in both sexes. The males may sometimes be dis tinguished from the females by slight differences in the appear ance of the genital opening. In many cases, however, the only difference, not always distinct, is the presence of little apertures of crural glands on the legs of the male (fig. 3).
The other apertures on the surface of the body are those of the tracheae (respiratory organs—see below) and the excretory organs. None of the former can be seen with the naked eye and only four of the latter, on the 4th and 5th pairs of legs (fig. 3).
The limbs of Peripatus are characteristic. Each consists of a cone-like stumpy leg bearing distally a narrower foot which carries two sickle-shaped claws. The skin of the legs bears rings of tiny papillae like those of the body and near the apex there may be spinous pads. The structure of the appendages is thus quite un like the jointed arthropod leg.
Internal Structur e.—A transverse section through an adult shows clearly how the body cavity, which is a haemocoele, is subdivided by delicate sheets of tissue (fig. 4) into (I.) a large central space extending the whole length and containing the gut (A), slime glands (B) and reproduc tive organs (c) ; (II.) a shallow dorsal space above the central cavity, containing the heart (n) ; (III.) two lateral spaces each with a nerve cord (E), nephridia (F), and salivary glands (s).
The excretory organs are also found in these compartments, i especially in the extensions which exist in the legs.
The Alimentary Canal. The buccal cavity contains a pair of horny jaws (fig. 5) which may just be seen from the exterior. Each consists of two cutting plates lying in contact. These jaws are the only mouth appendages present. The mouth at the pos terior end of the buccal cavity leads into a short muscular pharynx and from this a short oesophagus opens into the stomach (fig. 5). This forms the greater part of the alimentary canal. It is a straight and wide tube leading almost to the posterior end where a short, narrow rectum opens at the terminal anus.
The only glands connected with the alimentary canal are two salivary glands (fig. 5) which open by a common duct into the buccal cavity. Each gland is a long tubular structure lying in a lateral cavity close to the nerve cord.
The Slime Glands (fig. 5) are another peculiar feature. There are two of these, each opening on an oral papilla. Each consists of a long dilated tube which acts as a reservoir and lies over the stomach in the central cavity of the body. This reservoir extends back a considerable distance and then gives off numerous diverti cula in which the slime is secreted. These diverticula lie around the posterior part of the stomach, often entangled in the coils of the reproductive organs. When a living specimen is touched the slime is shot out from the oral papillae to a distance of several inches. Contact with the air causes it to congeal into white, sticky threads. It is supposed that the substance is used for offence or defence.
The tracheae are amongst the most noteworthy features of the Onychophora because so characteristic of certain other Arthro poda. They arise in bunches from the bottom of little epidermal pockets, tracheal pits. It is impossible, however, to see these pits externally. From each pit a large or small bunch of parallel tracheae start off, without branching; then they separate and finally branch, when they reach the organs they supply. Since the small single tubes require a high magnification to see them, one can only see readily the little rosettes of tubes where they are bunched at the tracheal pits and then only by examining the in ternal surface of the body-wall in fresh specimens.