=Relations of Peripatus to Insects.=—We will first recount the characteristics of this monotypic class. Peripatus (Fig. 4) stands alone, with no forms intermediate between itself and the worms on the one hand, and the true Arthropoda on the other. Originally supposed to be a worm, it is now referred to a class by itself, the Malacopoda of Blainville, or Protracheata of Haeckel. It lives in the tropics, in damp places under decaying wood. In general appearance it somewhat resembles a caterpillar, but the head is soft and worm-like, though it bears a pair of antenna-like tentacles. It may be said rather to superficially resemble a leech with clawed legs, the skin and its wrinkles being like those of a leech. There is a pair of horny jaws in the mouth, but these are more like the pharyngeal teeth of worms than the jaws of arthropods. The numerous legs end each in a pair of claws. The ladder-like nervous system is unlike that of annelid worms or arthropods, but rather recalls that of certain molluscs (Chiton, etc.), as well as that of certain flat and nemertine worms. Its annelid features are the large number of segmentally arranged true nephridia, and the nature of the integument. Its arthropodan features, which appear to take it out of the group of worms, are the presence of tracheæ, of true salivary and slime glands, of a pair of coxal glands (Fig. 4, _C_, _cd_) as well as the claws at the end of the legs. The tracheæ, which are by no means the only arthropodan features, are evidently modified dermal glands. The heart is arthropodan, being a dorsal tube lying in a pericardial sinus, with many openings. This assemblage of characters is not to be found in any marine or terrestrial worm. The tracheæ (Fig. 4, _D_, _tr_) are unbranched fine tubes, without a “spiral thread,” and are arranged in tufts, in _P. edwardsii_ opening by simple orifices or pores (“stigmata”) scattered irregularly over the surface of the body; but in another species (_P. capensis_) some of the stigmata are arranged more definitely in longitudinal rows,—on each side two, one dorsally and one ventrally. “The stigmata in a longitudinal row are, however, more numerous than the pairs of legs.” (Lang.) The salivary glands, opening by a short common duct into the under side of the mouth, in the same general position as in insects, are evidently, as the embryology of the animal proves, transformed nephridia, and being of the arthropodan type explain the origin and morphology of those of insects. It is so with the slime glands; these, with the coxal glands, being transformed and very large dermal glands. Those of insects arose in the same manner, and are evidently their homologues, while those of Peripatus were probably originally derived from the setiparous glands in the appendages (parapodia) of annelid worms. [Illustration: