194 PAUL S. WELCH. 



advantage evidently being due, in part at least, to the anchorage 

 which the emerging portion of the worm secures against the 

 surrounding materials with its setae. 



ABNORMALITIES. 



In the course of this work several different types of abnor- 

 malities were found, all of which appeared early in development 

 and were almost exclusively confined within cocoons since their 

 form was such as to prevent their escape. The following forms 

 were noted: (i) Whole body swollen and shorter than normal; 

 (2) anterior region swollen, posterior region normal; (3) local 

 swellings along the body; (4) bifurcations; and (5) combinations 

 of bifurcations with I, 2, and 3. Since this paper is concerned 

 only with the fourth type, the other forms of abnormality will 

 receive no further consideration. 



FREQUENCY OF BIFURCATION. 



During the past three years a large number of these bifurcated 

 worms have been found. While it is possible to examine a 

 cocoon and detect bifid individuals previous to any emergence, 

 they can be secured easily and in some quantity by allowing 

 cocoons to develop under as favorable laboratory conditions as 

 possible until most or all of the normal individuals have escaped. 

 Since the abnormalities only very rarely escape they are left 

 behind in the cocoons and are thus easily collected at a later 

 time. Records based upon more than five hundred cocoons 

 indicate that about twenty percent yielded bifurcated individuals. 



The maximum number of bifid individuals found in a cocoon 

 was three, one or two being more common. Thus far no relations 

 between the number of eggs per cocoon and the number of 

 bifurcations has been detected, the latter being about as frequent 

 in cocoons with few eggs as in those containing the larger num- 

 bers. 



FORMS OF BIFURCATION. 



The diversity of form of these bifurcations is striking and 

 demands some notice here. While an occasional irregularity 

 was difficult to interpret, most individuals could be classified 



