ONYCHOPHORA, a small but unusually interesting group of animals of the phylum Arthropoda (q.v.), differing in so many important respects from all other Arthropoda that a special class has been created for them. The class Prototracheata or Onychoph ora, containing only about 5o species, is equivalent in rank to the classes Crustacea, Insecta, Myriapoda and Arachnida, although these groups contain many thousands of genera and species.
A small group of genera which necessitates the creation of a separate class is usually the recipient of particular interest, for it will present highly important indications of the evolutionary rela tions between other groups of the animal kingdom. It is often to be regarded as the survivor of a group more extensive in range and more numerous in species and individuals in past times.
The Onychophora is such a case. It presents features which are typically arthropod. At the same time it possesses many features which recall the segmented worms (Annelida, q.v.), the group to which the Arthropoda are structurally most closely related. It might be regarded as a relic of the evolutionary transition between these big groups, the representative of an ancient group although probably it has evolved along its own special line.
The Onychophora contains only seven genera but these are so much alike that it is still common to use the term Peripatus as the generic name for all. The different species resemble each other externally so closely that, but for the differences in the number of legs, a picture in black and white like fig. I would stand for any of them. Notwithstanding this resemblance it ap pears necessary to restrict the use of the old generic name Peri patus to a few species.
The geographical distribution of the group is very wide but discontinuous and very local.
Specimens have been found in the West Indies, Central America and Chile, the Congo region of West Africa, South Africa, Malaya, India, Melanesia and Aus tralasia. Specimens are only met with here and there, even where favourable conditions exist over a wider area. This discontinuity of occurrence coupled with the obviously poor powers of distribu tion of these creatures is strong evidence of a more continuous range in the past and of a group now on the way to extinction. The
distribution of the genera is as follows :—Peripatus—America and Africa ; Eoperipatus—Indo-Malaya; Peripatoides and Ooperipa tus—Australia ; Opisthopatus—Chile and South Africa ; Para peripatus—New Britain ; Peripatopsis—Central Africa.
Since the present account is a short general summary we shall continue to use the term "Peripatus" to include all species of the group. It will be understood, however, that this is for convenience only.
The animal is always found in moist situations (although the district itself may not be moist during the whole year). It is generally found under rotting branches lying on the ground, under stones, under bark and in the crevices of tree stumps. It is ex tremely sensitive to a dry atmosphere (specimens frequently will not withstand 24 hours in a dry cardboard box, whereas they will be happy for twice this time in a small glass tube containing moist soil although the tube may be tightly corked). It never comes out into day-light and specimens in captivity are much more active at night. It is probably entirely carnivorous, feeding on small insects and other small animals.
Peripatus is a segmented animal, and at a first glance looks somewhat like a caterpillar; but the long antennae, the peculiar body-surface and the legs soon dispel that view. The actual seg mentation is only shown externally by the occurrence of paired legs, one pair to each segment. The surface of the body is marked by ring-like ridges far more numerous than the segments. Some of these are almost continuous round the circumference of the animal (they are all broken by a fine groove down the middle of the dorsal surface), others are less complete and arise between the former. The skin and body wall is highly characteristic, the superficial cuticle being very thin (as in the worms, in contrast to the usual arthropod condition) and raised everywhere on the ridges into delicate microscopic papillae. These close papillae give the skin a velvety ap pearance and their presence makes it difficult to wet the crea ture.