38 PRIMITIVE ANIMALS 



or faintly blue-coloured blood, the blue colour 

 being due to the presence of a respiratory substance, 

 haemocyanin, resembling haemoglobin but having the 

 iron replaced by copper. There is no distinction in 

 these animals between the body-cavity and the blood 

 vascular system, the body-cavity being, in fact, not a 

 coelom, but formed from greatly swollen veins which 

 surround all the organs of the body. What then has 

 happened to the true coelom ? We have seen that in 

 the coelomate Annelid two systems of organs are 

 always intimately related, both in function and 

 derivation, with the coelom, viz. the reproductive 

 and excretory organs : it is therefore natural to look 

 for the remains of the coelom of the Arthropods in 

 connection with these organs. The reproductive 

 organs of the Arthropods are hollow sacs derived 

 from the mesoderm with ducts leading to the exterior, 

 and on theoretical grounds we may suppose that these 

 sacs represent a part of the original coelom. Excretory 

 organs of various kinds are found in the Arthropods, 

 but they are not segmentally arranged ; we may men- 

 tion the shell and green glands found in Crustacea, 

 coiled tubes which end internally in hollow closed 

 sacs of mesodermal origin. It has been suggested 

 that these closed sacs represent a portion of the 

 coelom, and that the coiled tubes leading away from 

 them to the exterior are coelomoducts. Somewhat 

 similar organs are found in Arachnids. In the 



