28l The cuticle of Peripatopsis moseleyi By ELAINE A. ROBSON (From the Department of Zoology, University of Cape Town, and the Department of Zoology, University of Cambridge (present address)) With 9 plates (figs, i to 9) Summary The integument of Peripatopsis moseleyi has been examined by light and electron microscopy with particular reference to the structure and formation of the cuticle. The evidence supports the idea that Peripatus is a true arthropod but not that it has direct affinities with the annelids. The characteristics of arthropod cuticle are present in their simplest form and pore canals and dermal glands are lacking. The cuticle is 1 or 2fj, thick except in the hardened claws and spines. Above the procuticle (chitin- protein) is a thin 4-layered epicuticle. It is possible that the innermost of the 4 layers (prosclerotin) may correspond to cuticulin of other arthropods. In the claws and spines tanning in this layer extends to the procuticle. Hydrofuge properties of the cuticle probably depend on the outer layers of epicuticle, and it is suggested that the lamina concerned might consist of oriented lipid associated with lipoprotein (Dr. J. W. L. Beament). Wax and cement are absent. Non-wettability of the cuticle is probably ensured by the contours of micropapillae which cover the surface. Similar structures arise in Collembola and other terrestrial arthropods by convergence. The formation of new cuticle before ecdysis is described. After the epicuticular layers are complete, the bulk of the procuticle is laid down in a manner probably common to all arthropods. Secreted materials originate in small vesicles derived from rough endoplasmic reticulum and from scattered Golgi regions. The latter contribute to larger vacuoles which rise to the surface of the cell and liberate material in a fluid state. This later consolidates to form procuticle. Vesicles may also open to the surface directly, and ribosomes probably occur free in the cytoplasm. At this stage the cell surface is reticulate, especially under micropapillae. The ordinary epidermis has only one kind of cell, attached to the cuticle by tono- fibrils disposed like the ribs of a shuttlecock, and to the fibrous sheaths of underlying muscle-fibres by special fibres of connective tissue. These features and the presence of numerous sensory papillae are associated with the characteristic mobility of the body wall. The appearance of epidermal pigment granules, mitochondria, the nuclear membrane, and a centriole are noted. No other cells immediately concerned in the formation of cuticle have been found. By contrast myriapods, which do not have wax either, possess dermal glands secreting far more lipid than is found in the Onycho- phora. The wax layer found in insects and some arachnids constitutes an advance of high selective value which emphasizes the primitive condition of the Onychophora. It is noted that the thick layer of collagen separating the haemocoel from the epidermis probably restricts the transfer of materials. It is suggested that since some features of cuticular structure and formation appear to be common to all arthropods, it is possible that some of the endocrine mechanisms associated with ecdysis may also be similar throughout the phylum. Introduction THE evolutionary position of the Onychophora has been much discussed. A specimen collected by Sloane early in the last century was identified by Shaw as Nereispedata (Bouvier, 1905). Guilding (1826) considered Peripatus [Quart. J. micr. Sci., Vol. 105, pt. 3, pp. 281-299, 1964.] 2421.3 U 282 Robson—The cuticle of Peripatopsis moseleyi to be a mollusc, but most of his contemporaries referred it to the annelids or to a group linking the annelids and myriapods. Subsequent studies supported the latter view. The classic work of Balfour (1883) and Sedgwick (1885-8) suggested that although Peripatus had affinities with the annelids, it was evidently an arthropod. Chief arthropod features were the presence of hardened claws and jaws, a cuticle shed by ecdysis, the tracheal system, the large haemocoel, and dorsal ostiate heart. By contrast the extensible body with its smooth muscle and soft cuticle, serial nephridia, the presence of cilia, and the simple eyes were more characteristic of annelids. These observations supported the idea put forward on other grounds that the annelids and arthropods constituted a super-phylum (e.g. Lankester, 1904). The notion is established in several textbooks of zoology, where the Arthropoda are usually denned as a group with an annelid-like central nervous system, and Peripatus is regarded as a primitive link between the 2 phyla. Although Peripatus un- doubtedly has primitive features, it has been pointed out that many of its apparently annelid characters have an adaptive value which must affect their validity as phylogenetic markers (Manton, 1959). It is possible to relate many of them to the cryptic habits of the Onychophora rather than to the legacy of ancestral annelids. The group is still regarded by some as a distinct phylum (Snodgrass, 1938, 1952; Butt, 1959, i960), but it seems difficult not to accept the Onychophora as a class of the Arthropoda (Moseley, 1874; Sedgwick, 1885-8; Manton, 1950, 1953, 1958, 1961; Tiegs and Manton, 1958). Within this phylum they have most in common with the myriapods and insects. Recent embryological studies have failed to disclose any further annelid resemblances (Manton, 1949, i960; Butt, 1959, i960). On the other hand Tiegs (1940, 1945, 1947) described several features in the development of myriapods which suggest common ground with the Onychophora. It is beyond doubt that the Onychophora are an ancient group (e.g. Manton, 1949), and it has even been thought that their distribution indicates that of former land masses (Clark, 1915). The Cambrian fossils of Aysheaia represent marine Protonychophora (Walcott, 1911, 1931; Hutchinson, 1930), but un- fortunately they throw little light on phylogenetic problems. Living Onycho- phora hold great interest as primitive members of a special kind of forest fauna (Lawrence, 1953) and it is proposed to consider them in this light rather than to regard them as an inter-phyletic link. Previous work on the cuticle The integument consists of cuticle, epidermis, and dermal connective tissue (Balfour, 1883). Early authors refer to the cuticle as empirically 'chitinous' (Bouvier, 1905), but Kunike (1925) showed that the cuticle of Peripatus broelmanni gave a positive Schultze reaction. Manton (1938) recorded chitin and protein in the exuvium of Peripatopsis. The presence of chitin was con- firmed by Richards (1951), while X-ray analysis has indicated that it occurs as a-chitin as in other arthropods (Lotmar and Picken, 1949; Rudall, 1955, and see 1963). Robson—The cuticle of Peripatopsis moseleyi 283 Ecdysis was recorded by Hutton (1876) and has since been described by Steel (1896), Manton (1938), and Holliday (1942, 1944). A mid-dorsal split in the old cuticle starts behind the antennae and eventually extends the whole length of the animal. The cuticle is worked off from the front backwards, one appendage being freed at a time as from a multidigit glove. The process is assisted by contractions of the body and takes something over an hour (Lawrence, 1953). The lining of the anterior part of the gut is also shed (Manton, 1938). The crumpled pellet of cuticle is usually eaten (Steel, 1896). Accounts referring to Peripatopsis moseleyi, the species studied here, are those of Holliday (1942, 1944) and Lawrence (1953). Manton's careful observa- tions of 4 species of Peripatopsis established that moulting occurs every 2 weeks. The cuticle is shed at birth, and ecdysis continues throughout life. The endocrine mechanism responsible for moulting has not been studied in Onychophora. There may prove to be some parallel with apterygote insects, where ecdysis also continues throughout life (Boelitz, 1933; Paclt, 1956; Watson, 1962), or perhaps more especially with myriapods (Cloudsley- Thompson, 1958; Juberthie-Jupeau, 1963; Scheffel, 1961, 1963). Gabe (1954) has found neurosecretory cells in 3 onychophoran genera, but their functions are not yet known. Sanchez (1958) re-examined Peripatopsis moseleyi and suggested that the infracerebral organs may store neurosecretory products; but a possible homology with corpora allata of insects is still un- substantiated (von Kennel, 1885-8; Dakin, 1922). In captivity Onycho- phora may continue to grow and to moult for a number of years (Manton, 1938), but in natural conditions their activity fluctuates with seasonal climatic changes and it is not known to what extent feeding, growth, and other pro- cesses are reduced when winter temperatures are low. The shed cuticle is unhardened except for the claws, the jaws and their apodemes, and, to a lesser extent, sensory spines. The proximal surface is hydrophil, and the most crumpled specimen can be extended if it is floated on water (Steel, 1896; Holliday, 1944). The cast skin then exceeds the normal length of the animal by one-third (Manton, 1938). As it is not noticeably elastic the slack must normally be implicated in surface furrows. When cast the cuticle is white, but in position it is transparent and has no colour apart from the brown claws and jaws. The characteristic orange, black, or green pigments occur in the periphery of epidermal cells beneath (Balfour, 1883). In view of the cryptic habits of the Onychophora (Lawrence, 1953; Manton, 1958) it is not surprising that the sensory functions of the integu- ment are well developed. The whole body is covered with rounded or conical papillae which usually end in a sensory spine, and whose arrangement is a specific character (Bouvier, 1905-7). Corresponding to cells of the epidermis there are secondary papillae. Papillae, spines, and pigmentation are accen- tuated on the dorsal surface. The spines are sensitive to touch. Similar ones occur in large numbers on the antennae, where they are less robust, and on the sensory pads of the feet.