454 ZOOLOGY SECT. 



The brain and cesophageal connectives are formed in continuity 

 with the rudiments of the ventral nerve-cord. 



On the whole the development resembles that of Nereis, the 

 chief differences being such as may be traced to the non-occurrence 

 in the Earthworm of any free-swimming trochophore stage, with 

 the absence of such larval structures as the large pre-oral lobe, 

 the apical plate, the prototroch, and the larval nephridia or head- 

 kidneys. 



2. DISTINCTIVE CHARACTERS AND CLASSIFICATION. 



The Chaetopoda are Annulata with the body made up of distinct 

 metameres, which are usually numerous and similar throughout. 

 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, and not in com- 

 munication with the blood-vascular system, which is nearly 

 always highly developed. The ventral nerve-cord consists of a 

 chain of ganglia. The reproductive cells are formed by a prolifera- 

 tion of certain parts of the peritoneum or membrane lining the 

 coelome, and usually reach the exterior through ccelomoducts 

 or through modified or unmodified nephridia. 



Sub-Class I.-POLYCH^ETA. 



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 

 clitellum is never developed. A metamorphosis takes place : the 

 larva is a trochophore. Nearly all the Polychaeta are marine. 



ORDER 1. ARCHI-CMTTOPODA. 



Aberrant or primitive Polychaeta 1 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. PHANEROCEPHALA. 



Polychaeta with protrusible pharynx usually armed with chitinous 

 jaws. There is a well- developed head. The segments are com- 

 pletely or nearly similar throughout the length of the body, and 



1 The Archi-Chcetopoda are usually classed with the Polychceta, but their 

 alliancss are perhaps quite as close with the Oligochceta. In some respects 

 Saccocirrus resembles Polygordius and Protodrilus (Archi- Annelida, q.v.), 

 but is distinguished from them by the possession of setae. 



