NOTES ON REGULATION IN STYLARIA LACUSTRIS. 



E. H. HARPER. 



This paper is not concerned chiefly with regeneration, but 

 more especially with some processes of regulation accompanying 

 regeneration which were observed in the living animals. Stylaria 

 lacnstris L. is a well-known fresh-water oligochaete of the family 

 Naididae, whose members have the power of reproducing by self- 

 division. The worm is six to eight mm. in length, and is easily 

 recognized by its long prostomium or proboscis. The body is 

 divided into three regions : (i) An anterior specialized region of 

 five segments, containing the pharynx, which is marked by a yel- 

 low pigment distinct in appearance from the dark brown chlora- 

 gogue layer of the rest of the alimentary tract. The first seg- 

 ment bears the long prostomium and eyes. The mouth is ven- 

 tral and the pharynx is slightly eversible. The anterior region 

 is further distinguished by the absence of the capilliform dorsal 

 setae, ventral setae also being absent from the first segment. (2) 

 The middle region, with an indefinite number of segments, con- 

 taining the oesophagus and the crop, a dilatation between the 

 seventh and eighth segments, and the stomach-intestine. The 

 middle region ends in a budding zone of incompletely developed 

 segments and embryonic tissue. (3) The specialized anal seg- 

 ment, somewhat longer than the rest and tapering in form, devoid 

 of setae, and with the gut lined with cilia. 



Self-division takes place at a region posterior to the middle of 

 the worm, from the seventeenth to the twenty-fifth segments, 

 very commonly at the end of the twenty-first. The first indi- 

 cation of division is the appearance of a band of transparent 

 tissue divided in the middle by a slight constriction. Em- 

 bryonic tissue accumulates before and behind the septum between 

 the segments forming an ectodermal thickening. These regions 

 lengthen and soon become segmented, that in front of the septum 

 forming the anal segment of the anterior zooid, and the part 

 behind developing into a pharyngeal region of five segments for 

 the posterior zooid. The zooids remain united until completely 



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