POLYCHAETA 



159 



of the hinder part of the body. Genital ducts in the form of wide 

 ciliated funnels are likewise developed in many segments, but only 

 acquire openings to the exterior at the time of sexual maturity. 

 Sometimes they open into the tube of the nephridium. Such 

 combinations of nephridium and coelomiduct have been called 

 nephromyxia. The part of the animal containing the ripe sexual 

 cells sometimes breaks off from the head end and floats at the 

 surface of the sea. The head end may then grow a new series 



a a . 





FIG. 66. Transverse section through Nereis cultrifera, slightly simplified. 

 The parapodia are shown in perspective. Magnified. 



.. Cuticle. 2. Epidermis. 3. Circular muscles. 4. Longitudinal 

 muscles. 5. Oblique muscles forming a partition. 6. Parietal 



layer of epithelium. 7. Coeloin. 8. Visceral layer of peritoneum. 

 9. Cavity of intestine. 10. Dorsal blood-vessel. 11. Ventral blood- 

 vessel. 12. Ventral nerve-cord. 13. Nephridium cut in section. 

 14. Ova. 15. Notopodium. 16. Neuropodium.' 17. Dorsal 

 cirrus. 18. Ventral cirrus. 19. Chaetae. 20. Aciculum with 

 muscles at inner end. 



of hinder segments. The septa which divide the coelom in one 

 segment from that in the next are in many forms incomplete or 

 absent. 



As a rule Polychaeta have a certain number of the anterior 

 segments modified to form a head, which often carries tentacles; 

 organs for absorbing oxygen from the water, called branchiae 

 or gills are also often developed sometimes from the head some- 

 times from the middle region of the body. Polychaeta are generally 



