757 THE AMERICAN NATURALIST [VoL. LI 



these animals the two "rhinophores" themselves were separated 

 by their normal distance of about 1 cm. The next step in "rhin- 

 ophore" fusion is illustrated in Fig. 4, one example having been 

 collected. In another specimen the "rhinophores" were found 

 to be closely united at the base (Fig. 5), while in the remaining 

 two specimens that exhibit fusion of the "rhinophores" (Fig. 

 6) the process of coalescence had been pushed much further, a 

 single stalk, giving rise at its free end to two short diverging 

 projections, representing the normal pair of "rhinophores." 



When a "rhinophore" of C. zebra is locally stimulated by 

 being touched, it is retracted within its pocket, the basal collar 

 usually contracting over it, while the companion " rhinophore" 

 on the other side of the animal is usually not contracted. In 

 other words, the "rhinophores" are, with reference to their re- 

 traction, subject to independent bilateral control. The process 

 of retracting the "rhinophore" consists of two phases the 

 "rhinophore" is itself contractile, and it is in addition pulled 

 down into its pocket by the action of muscles situated at its base. 

 With the fused "rhinophores," even in such cases as that illus- 

 trated in Fig. 6, the independent bilateral control of the 

 organs is preserved. If one tip be stimulated, that side of the 

 compound "rhinophore" is contracted, the other (unless the 

 stimulation be severe) remaining inert. Under slightly stronger 

 stimulation applied to one tip of a compound "rhinophore," the 

 contraction of the organ itself is immediately followed by the 

 traction of muscles upon the same side of the base of the double 

 "rhinophore," resulting in a bending of the whole structure 

 toward the point of excitation. 



The reactions of the abnormal specimens therefore support 

 the view that these abnormal "rhinophores" have been pro- 

 duced by a process of fusion, probably resulting from the orig- 

 inal close approximation of "rhinophoral" Anlagen. Two cases 

 have been available for experiment in which one of the normally 

 placed "rhinophores" possessed a divided tip; these divided-tip 

 : ' rhinophores, " superficially not unlike the single median struc- 

 ture above described, gave no evidence of independent control 

 for the two tips, both parts contracting together when one tip 

 was irritated. 



It would appear that the development of the collar surround- 

 ing the base of the "rhinophore" is directly dependent upon the 

 growth of the latter structure; in every case there was a close 



