DOUBLE STRUCTURE IN PLANARIA LUGUBRIS. 137 



united to the other part by only a narrow connection. It con- 

 tains a pharynx, and a new tail has begun to grow out near the 

 area of attachment. This piece pulled apart later. Practically 

 the same results were obtained when the pieces were cut as 

 shown in Fig. 21. In this case the worm was first cut in two 

 near the old pharynx and then by means of an oblique cut, the 

 triangular piece was separated in such a way that a part of the 

 old pharynx was left in each piece. A new pharynx developed 

 in both pieces, and the same relations between the sizes of the 

 new heads, that were described in the first and third cases above, 

 hold here also. 



CONCLUSIONS. 



The results show clearly that the presence of a pharynx in a 

 new part is an important element in the subsequent growth of the 

 part. A part containing within itself a new pharynx behaves 

 more like an independent worm. The most natural interpreta- 

 tion would be, perhaps, that this is due to the part being able to 

 feed for itself; but while this may to some extent account for the 

 result, yet is probably not the whole explanation. My reason 

 for thinking so is that when the animal feeds the digestive tract 



d> Cj 



of the side piece is also filled with food material. If, therefore, 

 the planarian is kept well fed, the side piece does not lack food 

 material even where there is no pharynx in this part. 



There are some facts in connection with the mode of regenera- 

 tion of planarians that may throw some light on this lack of en- 

 largement of pieces without a pharynx. If the anterior end of 

 a worm is cut off, the new pharynx appears, in the anterior piece, 

 at the posterior edge of the old material. It is, at first structu- 

 rally, too, the head. If the piece is fed so that the old part loses 

 as little as possible, a new region is intercalated in front of the 

 pharynx, and this region continues to enlarge until the normal 

 proportions have been attained. Again, if a piece of Planaria 

 lugiibris is cut off behind the old pharynx, a new head arises at 

 the anterior end, and a new pharynx, also at the anterior end just 

 at the border between the new and the old parts. The head and 

 pharynx are in this case also too near together, but a new region 

 of growth is established between the head and the pharynx so 



