SECT. XVI PERIPATUS AND THE TRACHEATA 283 



not, we think, be far wrong in assuming that these 

 were developed as adaptations to a Hfe on land, and 

 appeared in the original Tracheatan-Annelid, in its 

 gradual passage from a purely aquatic to a terrestrial 

 life. It seems to be a strict biological law that, when 

 aquatic animals migrate to the land, external respira- 

 tory surfaces such as gills, which are morphologically 

 folds of the skin, give place to internal respiratory 

 surfaces. This requires no special comment. It is 

 probably, however, an equally strict biological law 

 that free movement on land necessitates such a place 

 of exit for the waste products as will not interfere with 

 such movement. Insects clean themselves from no 

 love of cleanliness. The disadvantages of discharging 

 the waste products in the cephalic or thoracic region, 

 as in the Crustacea, are avoided by means of the 

 Malpighian tubules which open into the hind-gut. 

 This is not the only advantage. Small land animals 

 have often to exercise the most rigid economy in their 

 supply of fluid. The discharge of the waste products 

 into the hind-gut permits the reabsorption of their 

 purely fluid constituents, which would thus be re- 

 tained within the body. These two advantages are of 

 such importance that the gradual concentration of 

 excretion to the walls of the hind-gut (which we saw 

 in Apus to be highly glandular) until special excretory 

 caeca, the Malpighian tubules, were developed, pre- 

 sents no difficulty. 



We have already referred to the able attempt of 

 several distinguished zoologists, Kingsley in America, 

 and Ray Lankester in England, to connect the Arach- 



