PAUL S. WELCH. 



MATERIAL. 



The material on which this paper is based was identified by 

 Piofessor Frank Smith as Tubifex tubifex (Mull.). Certain 

 localities near Ann Arbor, Michigan, furnish an abundant supply 

 of this annelid, and it has been possible, during the past three 

 years, to obtain large quantities of sexually mature worms and 

 cocoons from November until July either in the native environ- 

 ment or in cultures in the laboratory. Easy access to such an 

 extensive supply of material has made possible the large number 

 of examinations referred to in this paper. 



Owing to the abundance of material it was often possible to 

 secure cocoons easily by merely bringing masses of the mud into 

 the laboratory, spreading it out in large shallow dishes, covering 

 it with a small amount of water, allowing the finely divided 

 matter to settle, and gently working over the material with a 

 pipette. Fortunately the mud does not adhere to these cocoons 

 and they will usually appear clean and distinct when uncovered. 

 Furthermore, their color, which is usually of a very light orange, 

 is such as to offer distinct contrast to the mud in which they 

 occur and except for the rough resemblance to the small sand 

 grains sometimes mingled with the mud, the recognition of the 

 cocoons offers no particular difficulty. For securing large 

 numbers of cocoons quickly the writer has employed the method 

 of running quantities of pond and river margin materials through 

 a set of graded sieves, the finest one having a mesh just small 

 enough to retain the cocoons. The residue was then thoroughly 

 washed by a gentle stream of water and examined under 

 good light conditions. Undue handling of materials sometimes 

 results in the loss by the cocoons of their ability to maintain 

 perfectly clean external surfaces, fine particles tending to adhere 

 and thus render cocoons difficult to detect. 



Since this work was largely concerned with certain abnormal- 

 ities in development the cocoons were handled as carefully as 

 possible. That the sieve method of collection was not responsible 

 for the production of abnormalities was demonstrated by com- 

 paring the materials so collected with those taken simultaneously 

 from the same habitat by other methods. 



