Zoology 10th Edition by Stephen Miller,John Harley

Exercise 8

Most annelids are marine worms. They are characterized by the presence of parapodia with numerous setae on most body segments. Parapodia are used in locomotion and burrowing. Parapodia are reduced in size in some sedentarians and absent in others. Setae are similarly reduced. Annelids have diverse feeding habits ranging from deposit to filter feeding. Annelids have a closed circulatory system and a ventral nervous system including segmental ganglia and nerves. Excretion usually occurs via segmental metanephridia, which open through the body wall. Polychaetes are dioecious but may reproduce asexually. Sexual reproduction usually occurs via external fertilization and the development of trochophore larvae. Annelids within the clade Clitellata are monoecious and have direct development.
Why does the association of a capillary bed with a metanephridium provide more efficient waste processing as compared to the function of a protonephridium?