282 



ANNELIDA 



Fig. 451 'Diagram of parapodia 

 (Cambridge Natural History). A, 

 Nephthys : B, Amphinome ; C, Gly- 

 cera; D, Syllis; E, Leodice ; P, Phyl- 

 lodoce. 1, notopodium ; 2, neuropo- 

 dium ; 3, cirrus. 



Key to the orders of Chcetopoda: 



<*! Segmentation distinct. 

 bi Parapodia with complex groups of setae ; usually cephalic appendages 



present ; mostly marine 1. POLYCH^STA 



b a No parapodia or cephalic appendages present; mostly fresh-water or 



terrestrial 2. OLIGOCH^TA 



o, Segmentation indistinct or wanting in adult; marine 3. ECHIUBIDA 



ORDER 1. POLYCH-STA. 



Mostly marine annelids, either free- 

 swimming or sedentary, which are pro- 

 vided with parapodia. Two portions 

 may usually be distinguished in the 

 parapodium (Fig. 451), the dorsal no- 

 topodium and the ventral neuropodium, 

 both of which may contain setae; two 

 cirri, a dorsal and a ventral, extend 

 from it. In the sedentary polychaets 

 the parapodia are much reduced in 

 size on either the whole or a part of 

 the body and in a few they 'are entirely 

 wanting. 

 The polychaBts 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, 



and two palps which are ventral 



and in certain forms are broken 



up into long respiratory filaments. 



Elongated peristomial cirri may 



also be present. Eyes are often 



present on the prostomium and 



lithocysts are found in a few forms 



(Arenicola). The mouth is in the 



peristomium and is often provided 



with a proboscis. Polychaets are 



often highly colored; bright red, 



green, blue, and yellow tints char- 

 acterize many of them and make 



them very beautiful animals. 



Polychsets are usually born as 



trochophore larvae and pass through a metamorphosis before reaching 



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



Fig. 452 Diagram of the beads of 

 various polychaets (Cambridge Natural 

 History). A, polynoid ; B, syllid ; C, 

 Nephthys; D, Leodice; E, Phyllodoce; 

 F, Trophonia. 1, prostomium ; 2, peri- 

 stomium ; 3, tentacles ; 4, palps ; 5, 

 peristomial cirri. 



