TYPE ANNELIDA. 211 



In Capitella only one pair, that of the eighth metamere, becomes con- 

 verted into a genital duct; and it is interesting to note that in this same 

 segment a true excretory nephridium is also present. Whether this indi- 

 cates or not the occurrence originally of more than one pair of nephridia in 

 each metaniere remains to be seen, but it is interesting in connection with 

 what occurs in the Oligochseta (see p. 223). 



The reproductive organs consist of local thickenings of the 

 peritoneal epithelium (Fig. 99, ov) in more or fewer of the 

 segments. The ova or spermatozoa fall when ripe into the 

 ccelomic cavity and pass to the exterior by the nephridia. 

 With very few exceptions the Polychaeta are bisexual. 



The classification into smaller groups is to a certain ex- 

 tent artificial at- present, and does not profess to have any 

 phylogenetic significance. Three orders may be recognized. 



1. Order Archiannelida. 



This order includes a few forms which are supposed to 

 present more primitive structural characteristics than the 

 remaining Polychaets. They show as a rule but indistinct 

 traces of an external segmentation, and are entirely devoid of 

 either parapodia or setae. Tentacles occur at the anterior 

 extremity of the head metamere ; but no other appendages, 

 such as cirri or branchiae, occur. The nervous system is im- 

 bedded in the hypodermis, and the nephridia are short tubes, 

 a single pair occurring in nearly every segment. To this 

 group belong the genera Polygordius and Protodrilus. 



2. Order Errantia. 



In this order are placed the free-swimming or creep- 

 ing Polychaeta, in which a considerable similarity of the 

 various trunk rnetameres occurs. The parapodia are as a 

 rule well developed, and occasionally are broad and plate-like 

 in adaptation to a free-swimming existence. Branchiae are 

 usually found on the dorsal lobes of a considerable number 

 of parapodia ; the head is distinctly marked off from the 

 trunk and may bear eyes ; while the anterior portion of 

 the digestive tract is converted into a protrusible pharynx, 

 usually armed with chitinous teeth. To this order belong 

 the genera Nereis (Fig. 100), usually found lurking beneath 



