and becomes entirely transformed into a nephridium (fig. 12,
D, 2′) The dorsal part shifts dorsal wards and diminishes relatively
in size (fig. 12, C). Its fate differs in the different parts
of the body. In the anterior somites it dwindles and disappears,
but in the posterior part it unites with the dorsal divisions of
contiguous somites of the same side, and forms a tube—the generative
tube (fig. 12, D, 2) The last section of this tube retains its
connexion with the ventral portion of the somite, and so acquires an
external opening, which is at first lateral, but soon shifts to the
middle line, and fuses with its fellow, to form the single generative
opening. The praeoral somite develops the rudiment of a nephridium,
but eventually entirely disappears. The jaw somite also
disappears; the oral papilla somite forms ventrally the salivary
glands, which are thus serially homologous with nephridia. The
various divisions of the perivisceral cavity develop as a series of
spaces between the ectoderm and endoderm, and later in the mesoderm
The mesoderm seems to be formed entirely from the
proliferation of the cells of the mesoblastic somites. It thus appears
that in Peripatus the coelom does not develop a perivisceral portion,
but gives rise only to the renal and reproductive organs.
|
| (After Sedgwick.) |
|
Fig. 12.—A series of diagrams of transverse sections through Peripatus embryos to show the relations of the coelom at successive stages. |
|
A, Early stage; no trace of the vascular space; endoderm and ectoderm in contact. B, Endoderm has separated from the dorsal and ventral ectoderm. The somite is represented as having divided on the left side into a dorsal and ventral portion. C, The haemocoele (3) has become divided up into a number of spaces, the arrangement of which is unimportant. The dorsal part of the somite has travelled dorsalwards, and now constitutes a small space (triangular in section) just dorsal to the gut. The ventral portion (2) has assumed a tubular character, and has acquired an external opening. The internal vesicle is already indicated, and is shown in the diagram by the thinner black line: 1, gut; 2, somite; 2′, nephridial part of coelom; 3, haemocoele; 3′, part of haemocoele which will form the heart—the part of the haemocoele on each side of this will form the pericardium; 4, nerve-cord; 4, slime glands. D represents the conditions at the time of birth. The coelom is represented as surrounded by a thick black line, except in the part which forms the internal vesicle of the nephridium. |
The genus Peripatus was established in 1826 by L. Guilding, who first obtained specimens of it from St Vincent in the Antilles. He regarded it as a mollusc, being no doubt deceived by the slug-like appearance given by the antennae. Specimens were subsequently obtained from other parts of the neotropical region, and from South Africa and Australia, and the animal was variously assigned by the zoologists of the day to the Annelida and Myriapoda. Its true place in the system, as a primitive member of the group Arthropoda, was first established in 1874 by H. N. Moseley who discovered the tracheae. Peripatus is an Arthropod, as shown by (1) the presence of appendages modified as jaws, (2) the presence of paired lateral ostia perforating the wall of heart and putting its cavity in communication with the pericardium; (3) the presence of a vascular body cavity and pericardium (haemocoelic body cavity), (4) absence of a perivisceral section of the coelom. Finally, the tracheae, though not characteristic of all the classes of the Arthropoda, are found nowhere outside that group, and constitute a very important additional reason for uniting Peripatus with it. Peripatus, though indubitably an Arthropod, differs in such important respects from all the old-established Arthropod classes, that a special class, equivalent in rank to the others, and called Prototracheata or Onychophora, has had, as we have seen, to be created for its sole occupancy. This unlikeness to other Arthropoda is mainly due to the Annelidan affinities which it presents, but in part to the presence of the following peculiar features: (1) the number and diffusion of the tracheal apertures; (2) the restriction of the jaws to a single pair, (3) the disposition of the generative organs, (4) the texture of the skin; and (5) the simplicity and similarity of all the segments of the body behind the head. The Annelidan affinities are superficially indicated in so marked a manner by the thinness of the cuticle, the dermo-muscular body-wall, the hollow appendages, that, as already stated, many of the earlier zoologists who examined Peripatus placed it among the segmented worms; and the discovery that there is some solid morphological basis for this determination constitutes one of the most interesting points of the recent work on the genus. The Annelidan features are: (1) the paired nephridia in every segment of the body behind the first two (Saenger, Balfour); (2) the presence of cilia in the generative tracts (Gaffron). It is true that neither of these features is absolutely distinctive of the Annelida, but when taken in conjunction with the Annelidan disposition of the chief systems of organs, viz. the central nervous system, and the main vascular trunk or heart, they may be considered as indicating affinities in that direction.
Synopsis of Species.
Peripatus (Guilding).—Soft-bodied vermiform animals, with one pair of ringed antennae, one pair of jaws, one pair of oral papillae, and a varying number of claw-bearing ambulatory legs. Dorsal surface arched and more darkly pigmented than the flat ventral surface. Skin transversely ridged and beset by wart-like spiniferous papillae. Mouth anterior, ventral; anus posterior, terminal. Generative opening single, median, ventral and posterior. One pair of simple eyes. Brain large, with two ventral hollow a pendages; ventral cords widely divaricated, without distinct ganglia. Alimentary canal simple, uncoiled. Segmentally arranged paired nephridia are present. Body cavity is continuous with the vascular system, and does not communicate with the paired nephridia. Heart tubular, with paired ostia. Respiration by means of tracheae. Dioecious; males smaller and generally less numerous than females. Generative glands tubular, continuous with the ducts. Viviparous. Young born fully developed. Distribution: Africa (Cape Colony, Natal, and the Gaboon), New Zealand, Australia and Tasmania, New Britain, South and Central America and the West Indies, the Malay Peninsula [and in Sumatra?].
The genus Peripatus, so far as adult conformation is concerned, is a very homogeneous one. It is true, as was pointed out by Sedgwick, that the species from the same part of the world resemble one another more closely than they do species from other regions, but recent researches have shown that the line between them cannot be so sharply drawn as was at first supposed, and it is certainly not desirable in the present state of our knowledge to divide them into generic or subgeneric groups, as has been done by some zoologists. (The following genera have been proposed: Peripatus for the neotropical species, Peripatoides for the Australasian, Perzpatopsis and Opisthopatus for the African. Paraperipatus for the New Britain, Eoperipatus for the Malayan species, and Ooperipatus for the suppose oviparous species of Australia and New Zealand.) The colour is highly variable in species from all regions; it is perhaps more constant in the species from the neotropical region than in those from elsewhere. The number of legs tends to be variable whenever it exceeds 19 praegenital pairs; when the number is less than that it is usually, though not always, constant. More constant points of difference are the form of the jaws, the position of the generative orifice, the presence of a receptaculum seminis and a receptaculum ovorum, the arrangement of the primary papillae on the distal end of the feet, and above all the early development.
South African Species.—With three spinous pads on the legs,
