44 PRIMITIVE ANIMALS 



two, and of the two cavities so formed on each side 

 in every segment, the one travels up on to the 

 back above the intestine, the other remains below 

 (Fig. 7 B, c). As this division and migration occur, 

 the ectoderm and endoderm draw apart, leaving wide 

 spaces between them, and these are gradually con- 

 verted into the heart and the blood vascular system 

 of the adult which usurps the function of a body- 

 cavity. The true coelomic partitions on the other 

 hand, the divisions and behaviour of which we have 

 mentioned, do not enlarge to form the body-cavity as 

 in a worm. They remain comparatively small, and 

 the pieces that have travelled dorsally form the 

 reproductive organs, while those that have remained 

 in a ventral position acquire openings to the exterior 

 and are converted into the excretory organs, which 

 are, therefore, coelomoducts (Fig. 7 D). The develop- 

 ment of Peripatus proves that the body-cavity of 

 this animal is not a true coelom, but is part of the 

 vascular system, and it also proves that a true seg- 

 mented coelom is formed in the embryo just as in a 

 worm, the remains of which in the adult are to be 

 found in the reproductive organs and the segmentally 

 arranged excretory organs. 



Peripatus, therefore, is a real link between the 

 Annelid and Arthropodan type as regards this most 

 important feature, the relation of the body-cavity and 

 vascular system. In all its other characters, perhaps 



