1874.] and Development of Peripatus capensis. 349 



the less perfect concentration mesially of the nerve-cords in Scolo- 

 pendra. 



Peripatus thus shows affinities, in some points, to all the main branches 

 of the family tree of Tracheata ; but a gulf is fixed between it and them 

 by the divarication of the nerve-cords : tending in the same direction are 

 such facts as the non-striation of the muscles, the great power of extension 

 of the body, the arrangement of the digestive tract in the early stage, the 

 persistence of metamorphosis, and the nature of the parts of the mouth, 

 the full history of the manner of origin of these being reserved. 



There are many speculations as to the mode of origin of the tracheae 

 themselves in the Tracheata. Professer Hackel (' Biologische Studien,' 

 p. 491) follows G-egenbaur, whose opinion is expressed in his * Grundziige 

 der vergleichenden Anatomic/ p. 441. Gegenbaur concludes that 

 tracheae were developed from originally closed tracheal systems, through 

 the intervention of the tracheal gills of primaeval aquatic insects now 

 represented as larvae. If Peripatus be as ancient in origin as is here 

 supposed, the condition of the tracheal system in it throws a very 

 different light on the matter. Peripatus is the only Tracheate with 

 tracheal stems opening diffusely all over the body. The Protracheata pro- 

 bably had their tracheae thus diffused, and the separate small systems after- 

 wards became concentrated along especial lines and formed into wide main 

 branching trunks. In some forms the spiracular openings concentrated 

 towards a more ventral line (lulus) ; in others they took a more lateral 

 position (Lepidopterous larvae, &c.). A concentration along two lines of 

 the body, ventral and lateral, has already commenced in Peripatus. The 

 original Protracheate being supposed to have had numerous small tracheae 

 diffused all over its body, the question as to their mode of origin again 

 presents itself. The peculiar form of the tracheal bundles in Peripatus, 

 which consist of a number of fine tubes opening into the extremity of a 

 single short common duct leading to the exterior of the body, seems to 

 give a clue. The tracheae are, very probably, modified cutaneous glands, 

 the homologues of those so abundant all over the body in such forms as 

 Bipalium or Hirudo. The pumping extension and contraction of the body 

 may well have drawn a very little air, to begin with, into the mouths of the 

 ducts ; and this having been found beneficial by the ancestor of the Pro- 

 tracheate, further development is easy to imagine. The exact mode of 

 development of the tracheae in the present form must be carefully studied; 

 there was no trace of these organs in the most perfect state of Peripatus 

 which I obtained. 



Professor Gegenbaur's opinion on the position of Peripatus (' Grund- 

 ziige der vergleichenden Anatomie,' p. 199) is, that its place among 

 the worms is not certain, but that, at any rate, it connects ringed 

 worms with Arthropods and flat worms. The general result of the pre- 

 sent inquiry is to bear out Professor Gegenbaur's opinion ; but it points 

 to the connexion of the ringed and flat worms, by means of this inter- 



