CHAPTER XII 



CHAEACTERS OF THE SUB-ORDERS OF POLYCHAETES CHARACTERS 



OF THE FAMILIES DESCRIPTION OF BRITISH GENERA AND 



SPECIES THE MYZOSTOMARIA. 



Systematic. The Order Polychaeta may be divided into two 

 branches, in one of which, the PHANEROCEPHALA, the prostomium 

 retains its ancestral condition as a lobe overhanging the mouth, 

 and frequently carries, in addition to paired eyes, certain sensory 

 processes of a simple structure, the tentacles and palps ; the body- 

 segments are more or less alike, and (except in some Spioni- 

 formia, some of the Terebelliforraia, and the Capitellifdrmia) do 

 not present two sharply marked regions, owing to the differential 

 arrangement or character of the chaetae. In the second branch, 

 the CRYPTOCEPHALA, the peristomium grows forwards during 

 development, so as to compress or even hide the prostomium, 

 which thus becomes a very insignificant organ. The tentacles 

 are reduced, but the palps become greatly developed and take 

 on sundry new functions. The body in this group, by the 

 character and arrangement of the chaetae, is distinguishable into 

 a thorax and abdomen, presenting certain internal differences. 



These two branches may be supposed to have arisen from a 

 common ancestor having a general resemblance to a iiereidiform 

 worm, such as Syllis, possessing palps and tentacles on the pro- 

 stomium, definite parapodia and cirri on the body, and internally, 

 a well-marked and regular repetition of organs. 



The branch PHANEROCEPHALA contains the following five sub- 

 Orders, though it is possible that the Capitelliformia deserves a 

 more important position in the system : 



SUB-ORDER 1. The Nereidiformia have well- developed ten- 

 tacles and palps ; the peristomium almost invariably possesses 



