THE INSECTA 171 



whole body ; its anterior opening, the mouth, is overhung by 

 a prostomium; its posterior opening, the anus, opens on the 

 terminal segment. The nervous system consists of a supra- 

 cesophageal ganglion connected by cords passing round the 

 gullet with a ventral chain of ganglia. The principal blood- 

 vessel is dorsal in position. Though the ccelom has practically 

 disappeared, and its place has been taken by blood-spaces in 

 adult arthropods, we find paired ccelomic pouches well de- 

 veloped in the embryo of the cockroach. It seems evident 

 that in the arthropods the limbs or parapodia lying nearest to 

 the mouth have been modified to form tactile organs (antennae) 

 or masticatory organs, and that the segments to which they 

 belong have become more or less intimately fused with the 

 prostomium and peristomial segment to form a head. There 

 is some reason to believe that the green glands and shell- 

 glands of the crustacae are homologous with the nephridia of 

 worms, and though these organs are absent in insects (unless 

 indeed they may be represented by the Malpighian tubes), as 

 also in the air-breathing scorpions, spiders, and centipedes, 

 yet we have the instance of the remarkable arthropod, Peripatus, 

 which is a terrestrial animal breathing air by means of tracheae 

 and has nephridia opening at the bases of each of its numerous 

 pairs of legs. These questions, however, are among the most 

 debatable in the ^domain of comparative anatomy, and it 

 would be quite out of place to attempt to discuss them in an 

 elementary treatise. But even a beginner can hardly fail to 

 be impressed by the many indications of relationship revealed 

 by the study of the anatomy and embryology of such apparently 

 unlike animals as those which have been described in this 

 and the preceding chapters as examples of the ccelomate 

 invertebrata. 



