No. 2.] FRESHWATER AND LAND NEMERTEANS. 483 



number, — only a few individuals possessed four eyes, while 

 the majority showed five, six, seven, or eight. Now I consider 

 this variability in the number of the eyes of the freshwater 

 forms to be explained by the general law, that all organs (and 

 propter hoc all organisms) which are undergoing progressive or 

 regressive development, tend to be variable. There is a pro- 

 gressive development taking place in the eyes of the freshwater 

 nemerteans, with regard to their number, and hence the number 

 is variable, since the action of the progressive development is 

 still contitmmg. Now if this law, which of late years seems to 

 have been lost sight of by most zoologists, — the law that pro- 

 gressive or regressive development is always accompanied by 

 variability, can be shown to be of general application, and I 

 know of no case to the contrary ; then the deduction must 

 follow, that the amount of variability above or below a given 

 mean will stand in inverse ratio to the length of time in zvJiich 

 the development (^progressive or regressive) has acted upon the 

 give7i organ. Thus through a great lapse of time the agency 

 of the development producing four eyes in the marine genus 

 Tetrastetnma has acted, so that a variability in the number of 

 the eyes almost never occurs ; on the other hand, the develop- 

 ing agency tending to produce more than four eyes in the 

 freshwater nemerteans, has acted for a comparatively much 

 shorter time, and accordingly the variability in the number of 

 the eyes of these forms is very great. And from all this, it 

 may be possible, by comparing the relative amount of varia- 

 bility in the number of the eyes of marine and freshwater 

 forms, to deduce the comparative age of the latter. 



The freshwater nemerteans accordingly differ in the number 

 and variability of their eyes from the allied marine species, and 

 this also seems to be the case with the land forms ; but data 

 are wanting in regard to the structural modifications produced 

 by the change of environment in the other organs. Naturally 

 the food obtained in the freshwater would differ from that in 

 the sea, — thus the two marine species of Tetrastemma exam- 

 ined by me seem to feed mainly on Algae-spores and Infusoria, 

 while the freshwater Stichostevima eilhardi contain almost ex- 

 clusively small Crustacea. This difference in the food obtained 



