230 Royal Society : — 



tozoa, and in being viviparous, as are some Scolopendrce ; further, 

 in the position of the orifices of the generative glands and in 

 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 (' Biolo- 

 gische Studien,' p. 491) follows Gegenbaur, 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 con- 

 dition 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 probably 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 open- 

 ings 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 Protracheate, further de- 

 velopment 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 

 (' Grundziige 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 fiat worms. The 

 general result of the present inquiry is to bear out Professor 



