CHAPTEK X 



CLASSIFICATION OF THE POLYCHAETA SHAPE HEAD PAKAPODIA 



CHAETAE GILLS INTERNAL ORGANS JAWS SENSE 



' ORGANS REPRODUCTION LARVAL FORMS BUDDING FIS- 

 SION BRANCHING REGENERATION. 



The Polychaeta are marine worms whose bodies are usually 

 elongated and cylindrical ; they either lead a free life, swimming 

 in the open sea, or crawling along the bottom ; or they pass their 

 life in burrows or definite tubes of various kinds. 



Each segment is normally provided on each side with a single 

 or a couple of bundles of chaetae, by means of which locomotion 

 is effected. These, in the free-living forms, are carried at the 

 ends of lateral muscular outgrowths of the body, known as 

 " parapodia," which are practically limbs. 



The " head " of the worm generally carries eyes, and frequently 

 more or less elongated tactile organs, the " tentacles " dorsally 

 and " palps " ventrally. The foregut is frequently provided with 

 a masticating apparatus in its anterior region, which is capable 

 of protrusion ; but this apparatus is absent in many burrowing 

 and tubicolous forms. The sexes are separate, so that there is no 

 such complicated system of generative organs as occurs in the 

 Oligochaeta. The nephridia usually act as genital ducts. In the 

 majority of cases the egg develops into a larva, the " Trocho- 

 sphere," which leads a free life and undergoes a greater or less 

 metamorphosis into the adult condition. 



The classification of Polychaeta adopted in this work is as 

 follows : 1 



1 This is a modification of the classification proposed by me at the meeting of the 

 British Association at Oxford, 1894 (see Report, p. 696). For further characteristics of 

 these Orders and sub-Orders see below Chap. XII. Elders, " Die Borstenwiirmer, " 

 1864, gives a historical survey of the group, and enumerates the earlier classifications. 



VOL. II S 



