HI'. I. .1 7 'IONS OF fXSKCTS TO OTHER ARTHROPOD A 9 



Relations of Peripatus to Insects. We will first recount the char- 

 acteristics 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 some- 

 what resembles a caterpillar, but the head is soft and worm-like, 

 though it bears a pair of antenna-like tentacles. It ma} 7 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 integu- 

 ment. Its arthropodan features, which appear to take it out of the 

 group of worms, are the presence of tracheae, 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 tracheae, 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 tracheae (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 evi- 

 dently their homologues, while those of Peripatus were probably 



