No. 548] A CASE OF POLYMORPHISM 453 



The internal organs are of the same general character, 

 although more compact and filling more of the body space 

 than in the two larger forms, two of them, however, be- 

 ing notably different in development. 



First, the nephridia. I have not studied these in 

 stained preparations as I have in the other types, but, 

 using living material, I make out a decidedly variable 

 number of flame cells ; they range from as low as twenty 

 to nearly forty. It is at least plain that the number 

 averages much less than in the humped form, and but a 

 fraction of that in the campanulate. The difference 

 would seem to be plainly correlated with the general dif- 

 ferences in the size of the organisms. 



Singularly in contrast to the smaller nephridia is the 

 development of the contractile vesicle or bladder. This 

 is a small organ in both the humped and campanulate 

 types, but in the much more diminutive saccates it be- 

 comes very much larger, filling, when expanded, a large 

 part of the body cavity. I have examined considerable 

 numbers from diverse sources and reared under different 

 conditions, to make sure that this contrast was not acci- 

 dental. I find no exceptions to it. Like the number of 

 flame cells, this matter of size of the contractile organ, 

 relative to that of the body, has been hitherto considered 

 a specific difference. 



A final peculiarity of the saccate type, although a var- 

 iable one, is its tendency toward an excessively rapid 

 rate of parthenogenetic reproduction; it is usually 

 crowded with embryos. Four, five and six arc frequent ; 

 sometimes I have counted nine; while in umny individuals 

 they are so closely packed that counting is out of the 

 question. These numerous young are also frequently 

 born at a disproportionately small size, even compared 

 tt-ith the diminutive parent, thus still farther increasing 

 the extremes of size which the species presents. It seems 

 probable that this smaller and less developed type is 

 itself, to some extent, a product of this rapid rate of 

 multiplication. I have found it multiplying at a slower 



