282 ANNELIDA 



Key to the orders of Chcetopoda: 



Ui Segmentation distinct. 

 bi Parapodia with complex groups of setae ; usually cephalic appendages 



present ; mostly marine 1- PoLYCHiETA 



&3 No parapodia or cephalic appendages present; mostly fresh-water or 



terrestrial 2. Oligocii^ta 



a. Segmentation indistinct or wanting in adult; marine 3. Echiurida 



Order 1. POLYCH-ffiTA. 



, 3 , , ^ Mostly marine annelids, either f ree- 



^^Q---^ ^pn" tj^ swimming or sedentary, which are pro- 



^yi « j 6 ^ c ^ vided with parapodia. Two portions 



^^J^"^ ^ y^*^ may usually be distinguished in the 



'i x * ■ t parapodium (Fig. 451), the dorsal no- 



^*5^ 5 ^V^ ( / ^^ topodium and the ventral neuropodium, 



,J!\/' ^^^^^'^""'"'">v. ^^ ^ ^^^^ ^^ which may contain setae ; two 



^ ^ ^ ■^^ t ^Y^^^^ cirri, a dorsal and a ventral, extend 



-**' V ^""''^ from it. In the sedentary polyehaets 



,J'^K •''a^~^''^^^^^ ^K^^^^^'^^'^t the parapodia are much reduced in 

 (Cambridge Natural History). A, if f 



Nephthys; B, Amphinome ; C, Giy- gi^e on either the whole or a part of 

 cera ; D, Syllis; E, Leodice ; F, Phyl- ^ 



lodoce. 1, notopodium ; 2, neuropo- the body and in a few they are entirely 

 dium ; 3, cirrus. -^ *' *' 



wanting. 



The polychaets have a distinct head which is usually provided 

 with special sense organs (Fig. 452). The prostomium may bear ten- 

 tacles, which may number from one 

 to ten and are dorsal in position, \hA'^ 5 



and two palps which are ventral Mm/U 



and in certain forms are broken 

 up into long respiratory filaments. 



Elongated peristomial cirri may A ^ \/ B 



also be present. Eyes are often 

 present on the prostomium and l^j "^ c 



lithocysts are found in a few forms MS^L^ , s llllllB 



(Arenicola). The mouth is in the 



peristomium and is often provided ^ ^ -^ ^ ^^^ . ^ 



with a proboscis. Polychaets are ^^W^ 



often highly colored; bright red,^ pj^ 452— Diagram of the heads of 



P-rppn hlnp nnrl vpIIow tint«? char- various polychaets (Cambridge Natural 



green, oiue, ana >eiiow iinis cnar- History). A, polynoid ; B, syiiid ; C, 



acfprizp innnv of tlipm inrl make Nephthys; D, Leodice; E, Phyllodoce; 



acieiize many ot mem ana make ^ Trophonia. 1, prostomium; 2, pen- 



them very beautiful animals. pSomi^rclrrL ''*'''''^' ' 4, palp.s; 5, 



Polych.'vts are usually born as 



trochophore larvae and pass through a metamorphosis before reaching 



the adult stage. Many reproduce asexually by serial or even lateral 



