396 ALLAN HANCOCK PACIFIC EXPEDITIONS VOL. 10 



(usually oil immersion) required to discern these 3-dimensional objects 

 implies that usually only thin optical sections are seen at any one time. A 

 single, long, curved tooth can thus be made to resemble several teeth in 

 ascending series by merely changing the depth of focus. Also, since the 

 hooks are highly translucent, any superimposed or underlying objects can 

 further distort the true outlines. The minuteness of these structures 

 enhances the possibility of aberration or misinterpretation. The smallest 

 species discussed below, (Capitita ambiseta, new species), has hooks in 

 which the beak is only about 2 micra across; a single tooth, of which 

 there are 3 in a row, thus measures less than one micron across. 



The microscopic details of these hooks have been carefully worked 

 out and illustrated for 19 species (see below). The parts of a hook are 

 herein designated as follows (pi. 43, figs. 1,2). The long embedded stem 

 is the shaft with the node near its middle length. It extends distally, 

 often forming a thickened shoulder with a slender neck and terminates 

 distally in a hooked beak provided with a larger fang surmounted by a 

 crescent composed of smaller teeth or denticles. The beak is more or 

 less loosely vested with a hyaline hood or sheath (rarely absent, in 

 Protocapitella) ; this is capelike, with an oval aperture distally to allow 

 for the protrusion of the teeth; the aperture is continued basally as a 

 cleft that extends nearly to the basal end of the hood. The hooks are 

 translucent yellow to slightly brownish, and the hood is translucent 

 yellow in all instances where these parts have been examined. 



The slender pointed setae are provided with a pair of limbate wings 

 at their end. These wings are probably homologous with the hyaline 

 hood of the uncini (see also Eisig, 1887, p. 105). The modified genital 

 hooks of Capitella and the terminal acicular spines of Scyphoproctus are 

 perhaps transformed pointed setae rather than hooks. Specialized setae or 

 hooks may function as holdfast organs (Scyphoproctus and Pulliella) or 

 for reproduction (Capitella). Some of the terminal segments may be 

 modified to form an anal plaque or funnel (Heteromastides and 

 Scyphoproctus). 



Sexes are separate, but sexual dimorphism is not conspicuous. In a 

 few cases (Capitella) the presence of specialized spines in posterior 

 thoracic segments distinguishes the two sexes. Copulation is known to 

 occur (Capitella and Notomastus), but artificial fertilization is possible 

 (see Wilson, 1935, p. 512). In some species ova are extruded through 

 modified nephridial apertures (Dasy bronchus). On the other hand, in 

 some other species (Clistomastus) it is likely that autotomy of posterior 



