Sub-Class I. POLYCILETA. 



x PHYLUM ANNULATA 427 



The metameres are provided with chitinous setae developed in sacs 

 (setigerous sacs) of the epidermis, and usually elevated on muscular 

 appendages, the parapodia. There is a large ccelome divided 

 internally into chambers by transverse septa or mesenteries, and 

 not in communication with the blood-vascular system, which is 

 nearly always highly developed. The ventral nerve-cord consists 

 i of a chain of ganglia. The reproductive cells are formed by a 

 proliferation of certain parts of the peritoneum or membrane lining 

 the ccelome, and usually reach the exterior through modified 

 or unmodified nephridia. 



Chaetopoda with the sexes distinct and the ovaries and testes of 

 simple character and metamerically repeated. Highly developed 

 parapodia are present, in most instances, bearing numerous long 

 setae. There is usually a definite head with eyes and tentacles, 

 and often cirri and branchiae on the segments of the body. A 

 metamorphosis takes place : the larva is a Trochosphere. All the 

 Polychaeta are marine. 



ORDER 1. ARCHI-CH^ETOPODA. 



Aberrant or primitive Polychaeta l in which the nervous system 

 is not separated from the epidermis and the ventral cord is not 

 segmented into ganglia. Only one genus (Saccocirrus). 



ORDER 2. ERRANTIA. 



Carnivorous free Polychaeta with protrusible pharynx usually 

 armed with chitinous jaws. There is a well-developed head. The 

 segments are completely or nearly similar throughout the length 

 of the body, and the parapodia are equally developed throughout, 

 and provided with cirri. The branchiae, when present, are not 

 confined to the anterior end. 



ORDER 3. SEDENTARIA. 



Vegetable-feeding Polychaeta which permanently inhabit tubes ; 

 devoid of protrusible pharynx and of jaws or teeth. The head is 

 frequently very small, and sometimes devoid of eyes or of tentacles. 

 The body is distinguishable, by differences in the form of the seg- 

 n"nts and of the parapodia, into two or even three regions. Thfex 

 parapodia are little prominent in the posterior parts, and usually/ 

 without cirri. The branchiae, when present, are usually confined 

 to the anterior end, and are sometimes represented by modified 

 cephalic tentacles. 



1 The Archi-Chcetojwda are usually classed with the Pclychceta, but their 

 alliances are perhaps quite as close with the Olifjochfrfa. 



