The tracheal pits are numerous and arranged in definite posi tions. In the West Australian species there are two main rows, one on each side, between the mid-dorsal line and the level of the legs. There are also four longitudinal series on the ventral surface. E. Gaffron stated that there were about 75 openings to a seg ment in P. edwardsi. The present author has counted 32 without difficulty in Peripatoides occidentalis. The tracheal tubes have a spiral thickening in the walls similar to that in other Arthropoda.
The excretory organs are probably the most surprising of all the structures found in this animal and at once call to mind the annelid worms, being not at all like the excretory organs of the arthropod groups. They are paired structures, one pair to each pair of legs. With the exception of those of the 4th and 5th pairs, each excretory tube opens on the ventral surface at about the point where its corresponding leg joins the body. The 4th and 5th pairs open on papillae near the distal end of the corresponding legs (fig. 3). It is a striking fact that this distinction should be found constantly in the different species.
The excretory organs have been termed nephridia and there is some evidence from embryology in support of this. Anatomical study, however, is insufficient to indicate the exact homologies of these organs and the embryological evidence requires confirmation. Each "nephridium" (fig. 6) consists of a tube with an enlarge ment, the collecting vesicle, near its aperture and another, cor responding to a coelomic cavity, at its internal blind end. In the West Australian Peripatoides the author discovered that cilia were present in that part of the tube leading out from the coelomic vesicle. This is the second part of the body where cilia have been found in these animals and since cilia are characteristically absent in the Arthropoda, it is a point of importance. Their other site is the reproductive organs of the female.
The blood system appears to consist only of a feebly developed dorsal- tubular heart.
The nervous system consists of a pair of large supraoesophageal ganglia completely united in the middle line and occupying most of the head space, and a pair of cords which run backward. These latter, instead of being close together in the mid-ventral line, are situated at the sides in the lateral cavities of the haemo coele. Numerous commissures, 8-10 between successive pairs of legs, connect the lateral cords. There are indications of ganglia, corresponding to the legs in number, where the lateral cords are slightly thickened. Nerves pass out from the supraoesophageal ganglia to the tentacles, eyes and skin sense organs and also leave the lateral nerve cords.
The only sense-organs are the eyes and numerous skin sense organs all of which look alike in structure.
The eyes are moderately well-developed, situated on the head near the base of the antennae. They are simple eyes, not com pound like those so characteristic of the Arthropoda. They differ also fundamentally from the arthropod type of simple eye, the ocellus. It is sufficient here to point out that on the whole the visual organ presents features of a simple type met with in both annelid worms and arthropods, but it has followed its own line of evolution.
The skin sense-organs consist of little packets of cells associated with a projecting spine. They look like and are usually regarded as tactile organs. Peripatus is, however, extremely sensitive to chemical stimuli as well as to vibrations and the presence of a very little chloroform vapour or acetic acid in the air produces a quick reaction.
One feature of special note is the varying amount of yolk in the eggs and its result. The Australian species have large yolk-laden eggs, and all species come near to laying eggs. At least one species of eastern Australia is actually oviparous and eggs are laid in sculptured shells. In all others the eggs are retained within the female and after a period of several months the young are born.
The South African and Australian species give birth to the young in April–June. Fertilization in one of the latter species takes place in the preceding August or September and the period of gestation would thus be 8-9 months. It has been given as 13 months for the Cape species.
The eggs are fertilized internally and it has been stated that the male of the Cape species deposits its spermatozoa on the surface of the female. Since the uterus is said never to contain sperma tozoa, the mode of entrance into the body would be a complete mystery. There can be no doubt that in the Australian species copulation of the two sexes takes place and the spermatozoa pass up the vagina of the female.
The embryology of the Cape species has been worked out and reference should be made to A. Sedgwick (see below) for details. Further work is required in this direction and would be of undoubted interest and importance.