They have a primitive protonephridial excretory system with waste products impelled along excretory canals by means of 
(They; have; a primitive protonephridial excretory system with waste products) (waste products; impelled; L:along excretory canals; by means of)
They waste products
have impelled
a primitive protonephridial excretory system with waste products
L:along excretory canals by means of