COPULATION IN A TURBELLARIAN WORM. 



299 



or whether the spermatozoa may not be deposited in the atrial 

 cavity of each worm and an exchange of them be effected by 

 simple apposition of the pores, as is essentially the case in 

 Annelida, also hermaphrodite. 



Contrary to the behavior of oligochaete annelids which adhere 

 in pairs, with anterior ends pointing in opposite directions, two 

 Planaria maculata mate with heads in the same direction (Fig. i). 



FIG. i. Attitude assumed by two Planaria maculata during copulation. 



The anterior ends are maintained side-by-side and flattened on 

 the supporting substance (bottom of watch-glass in this case), 

 oriented alike. About one third of their length posteriorly there 

 begins a rather slight spiral twisting of the flat, oral surfaces of 

 the worms against one another, so that the left ventral side of the 

 right worm of a pair becomes lifted up against the right ventral 

 side of the left worm. This twisting is carried further, posteriorly, 

 so that the tail ends of the worms may even cross one another. 

 At a point on the dorsal surface of each, opposite the external 

 opening (genital pore) of the atrium, there was a marked depres- 

 sion caused by the extension of the penis directly underneath. 

 This indicates that the penis of each worm is drawn into the 

 atrial cavity of the copulating mate by a definite muscular grasp 

 on the part of the walls of the enveloping atrium. This is also 

 suggested by the narrowing of the proximal end of the extended 

 penis; this is evident in Fig. 2. The relation of the two copulants 

 is thus presumably as in Fig. 2 ; this is purely a diagram, however, 

 and one may not infer that the penes necessarily lie laterally to 



