192 THE ANNELIDA POLYCHAETA. 



making possible their easy recognition. The body is elongated and cylindrical, 

 or but moderately depressed. The somites are numerous and in the ordinary 

 or atokous stage all are similar, excepting the usual modifications in the terminal 

 ones. 



The prostomium is distinct and well formed. It bears two pairs of eyes, 

 or these may rarely be absent. There is one pair of tentacles, and a pair of 

 palpi which are characteristically massive and two-jointed, with the terminal 

 joint short. 



The peristomium bears two pairs of tentacular cirri and may rarely bear 

 parapodia and setae also. 



The parapodia in general are biramous, though the most anterior one or 

 two pairs may be uniramous and the most posterior ones may be modified from 

 the prevailing type. The notopodium is occasionally rudimentary (Namaner- 

 eis and Lycastoides) . Notocirri and neurocirri present. 



The setae are composite and occur in two forms as to the character of the 

 articulation, namely the unsymmetrical, or heterogomph, and the synmietrical, 

 or homogomph, types. Usually both kinds occur at the same time, but not 

 always, one type in some forms prevailing exclusively. Stout spines, or 

 crochets, rarely present (Uncinereis, gen. nov.). 



The nephridia consist of long, convoluted tubules, each of which ends 

 internally in an open ciliated funnel, or nephrostome. 



The pygidium in general bears two anal cirri. 



The proboscis in extrusion presents two distinct regions or rings, a distal, 

 or maxillary, ring and a basal one. Each of these is again more or less clearly 

 divisible into six areas, three dorsal and three ventral, which are indicated in 

 descriptions as follows : — the median dorsal maxillary area is I, each lateral 

 dorsal maxillary area II; the corresponding ventral areas of the maxillary ring, 

 III and IV respectively; the corresponding areas on the basal ring above are V 

 and VI, and below VII and VIII. The proboscis always bears distally a pair 

 of maxillae curved toward each other and dentate on the mesal edge. The 

 surface of the proboscis otherwise may be smooth, or it may bear upon all or 

 part of the areas small, soft or hard papillae, or teeth, the paragnatha, which are 

 of much significance in classification. 



While some species of nereids reproduce without changing their ordinary 

 asexual body-form, many more undergo a pronounced metamorphosis affecting 

 both the external and the internal structure at the time of sexual maturity 

 and at the same time assume a pelagic life. The epitokous form, in general, is 



