424 WILSON. [Vol. VI . 



In the perianal area a long, stiff flagellum {a.f}) appears in the 

 middle line, and on either side of this one or two knobbed hairs, 

 like those of the apical region. As the anal cirrhi grow forth, 

 these knobbed hairs are borne at their tip (Fig. 91), where they 

 remain up to the latest stages I have observed. In connection 

 with these sense-organs, mention may also be made of a series 

 of pigment spots that appear on either side of the latero-dorsal 

 surface, about the middle of each somite. 



In the three-segmented stage, the larva remains for a relatively 

 long period (about twelve days), during which the only impor- 

 tant external changes concern the development of the head 

 (Fig. 92). 



In the first place, the head changes its form, becoming at 

 first flattened (Fig. 91) and afterwards elongated, tJie frontal 

 bodies bemg borne at its anterior extremity, while the prototroch 

 becomes interrupted above and below, so that only two lateral 

 tufts of cilia remain. At the same time the head becomes dis- 

 tinctly marked off from the trunk-region by a constriction that 

 lies immediately a^iterior to the first pair of parapodia, and poste- 

 rior to the tentacular cirrhi, and therefore in the lower hemisphere 

 of the trochophore. At a later period, a distinct segment-line 

 encircles the body at this point (Fig. 92), and the head is com- 

 pletely marked off. The mouth lies far anterior to this and in 

 front of the tentacular cirrhi, and there is no trace of a division 

 between the head and a mouth-segment ("mund-segment "). The 

 mouth-segment, therefore, belongs to the unsegmented part of 

 the body, and arises morphologically as a differentiation of the 

 head, — a result in entire accordance with my studies of Ln7n- 

 bricus. It is to be observed, further, that to the head-region 

 belongs, not only the upper hemisphere, but also the prototroch 

 and a portion of the lower hemisphere. 



Meanwhile the frontal bodies undergo certain changes of 

 form and finally disappear. They are at first spherical, but 

 afterwards assume a pear-shape, the clear space extending out 

 into the narrower portion, which is perhaps to be regarded as a 

 kind of duct. At first separated from each other, they are later 

 brought into contact, crowded closely together, and, after dimin- 

 ishing in size and becoming distorted in form, they disappear in 

 the surrounding ectoblast. 



Meanwhile the tentacular cirrhi grow out into long, slender 



