288 MENSCH. [Vol. XVI. 



In the head the anterior eyes have shifted more ventrally 

 and now lie almost beneath the posterior pair. A new pair of 

 tentacles, the posterior lateral (/./.), have made their appear- 

 ance simultaneous with the appearance of the swimming setae. 

 These are inserted anterior to the dorsal eyes and consist of 

 bud-like processes that extend a little beyond the margin of 

 the head. The buccal segment has become more prominent 

 and can be more readily distinguished from the region consti- 

 tuting the head. In a lateral view of the anterior part of this 

 and the lateral portions of the preceding stolons, it would be 

 noticed that the ventral surface of the anal segments of the 

 preceding stolon no longer lies in the same plane, but that the 

 buccal segment and even ventral parts of the succeeding seg- 

 ment are elevated considerably above the plane of the anal 

 segment, thus causing the attachment of this stolon to the 

 preceding one to appear to be more on the dorsal surface of 

 the anal segment than posterior, as in the preceding unions. 

 This is made very evident in specimens that are in a state of 

 partial contraction, but is also well marked in the normal state 

 and is due to the increase in size of the head, which, thicken- 

 ing only dorsally, pushes ventralward the tissues constituting 

 the zone of separation. 



After this stage of development has been attained the inde- 

 pendent movements of the stolon become very active and sep- 

 aration from the chain soon takes place. The separation of 

 the stolon takes place in the region of separation previously 

 referred to and which already prior to this stage, as will be 

 seen in the examination of sections of this region, has under- 

 gone a ventral degeneration by which this structure is gradu- 

 ally weakened so that when the stolon becomes more active, 

 rupture of the tissue is easily effected. 



The stolon when liberated from the chain has attained the 

 size of a mature free-swimming stolon and possesses all the 

 appendages, most of which have attained their full size and 

 undergo but few external changes in becoming completely 

 mature. The most important change concerns the carrying of 

 the anterior eyes exactly ventral to the posterior pair. This 

 process seems to be completed immediately after the separation 



