THE APPENDICULATE PHYLUM 39 



Centipedes and Insects a new type of excretory 

 organ is met with, the Malpighian tubules ; these 

 are numerous tubes which open into the hinder part 

 of the alimentary canal, and though it has been 

 suggested that they represent nephridia, it seems 

 more probable that they are entirely new and inde- 

 pendent excretory organs, and that the true nephridia 

 and coelomoducts have been lost together with the 

 portion of coelom into which they originally opened. 

 The fundamental difference in the relations of the 

 body-cavity and blood vascular system between the 

 Annelids on the one hand and the Arthropoda on the 

 other, is clearly a difference of great physiological and 

 morphological importance. The physiological reason 

 for the disappearance of the coelomic body-cavity 

 in the Arthropoda and its retention in the Annelids 

 and Vertebrata has never been properly worked out. 

 The only explanation we are prepared to offer is that 

 in soft-bodied animals like worms and vertebrates, 

 where the skeleton, if present, is situated internally, 

 the coelom with its fibrous and muscular walls exer- 

 cises an important mechanical function in protecting 

 the internal organs from rupture by external pres- 

 sure or by the strain of the muscular system upon the 

 skeleton. The internal organs and especially the 

 intestines are thus protected by being enclosed in a 

 double fibrous bag, the coelomic fluid between the 

 two layers of the bag acting as a lubricating oil. In 



