THE APPENDICULATE PHYLUM 35 



and Crustacea 1 . They further resemble these animals 

 in having the body composed of a number of seg- 

 ments, marked externally by rings; they are all 

 metamerically segmented animals. But if we open 

 any segmented worm by an incision along the middle 

 dorsal line, and compare what we see with an Insect 

 or Crustacean treated in a similar way, very great 

 and fundamental differences will be apparent. In an 

 Annelid such as an Earthworm we find that we have 

 cut into a body-cavity which is divided into as many 

 partitions as there are external rings or segments, 

 and in this segmented body-cavity the various organs, 

 such as the alimentary canal, the nerve-cord, the 

 excretory and reproductive organs lie. The body- 

 cavity or coelom contains a colourless fluid, the 

 coelomic fluid, in which amoeboid corpuscles float, 

 and it has no connection at all with the blood vascular 

 system which will be at once distinguished as a 

 number of vessels of various sizes containing a red 

 fluid, the blood, and distributed all over the body. 

 The blood-vessels form a completely closed circuit 

 in which the blood circulates, and nowhere does this 

 system communicate with the coelom or body-cavity. 

 It has been stated that the various organs lie in 

 the body-cavity. This statement is not strictly true. 

 If reference be made to Fig. 3, illustrating a transverse 

 section through the body of a worm, it will be seen that 



1 For classification of Appendiculata see Appendix A, pp. 150-1. 



32 



