286 MENSCH. [Vol. XVI. 



which is still visible in a lateral view of a stolon of this age. 

 On comparing it with the same region of younger stolons, it 

 will be noticed that it has become more constricted and forms 

 a much narrower bond of union between the two stolons, the 

 constriction at this stage being noticeable both ventrally and 

 laterally. It is at this stage of development also that the young 

 stolon shows the first indication of independent movement. 



Stolon 5 represents a stage in which the stolon has attained 

 its fullest length and breadth, and in which the general contour 

 of the Polybostricus is becoming more apparent. The most 

 striking difference between this and the preceding stolon exists 

 in the large increase in the size of the parapodia, particularly 

 in the middle region of the body, the development of the para- 

 podia being so great as to give the segments an entirely differ- 

 ent appearance in a surface view. If the anterior or posterior 

 surface of such a parapodium (PI. XIII, Fig. 12) be examined, 

 it will be noticed that the dorsal ramus (^.r.) has increased 

 very much in length and slightly exceeds the ventral ramus 

 both in length and breadth. The two rami remain fused in 

 the line of junction and thus form a broad, flattened appendage 

 with a tuft of setae protruding from the ventral angle. In the 

 outward growth of the dorsal ramus the dorsal cirrus has been 

 carried outward from its position on the side of the body-wall, 

 and now occupies a position on the dorsal angle of the para- 

 podium. As compared with the more simple parapodium of 

 the parent stock (PI. XIII, Fig. \a), this parapodium is not 

 alone very much larger, but by the outward growth of the 

 dorsal ramus the position of the ventral ramus has been so 

 changed^ as to cause it to assume a more transverse position 

 instead of inclining ventralward, as it does in the parent stock. 

 Normally, when at rest these parapodia are directed slightly 

 backward, thus already assuming the position of the parapodia 

 of the free forms. The parapodia of the first three setigerous 

 segments, however, do not undergo so complete a change. 

 They remain considerably smaller, and instead of being di- 

 rected backward, are placed directly at right angles to the 

 axis of the stolon, the growth of the dorsal ramus being con- 

 fined to an elongation of that region sufficient to carry the 



