234 MICROSCOPY FOR BEGINNERS. 



what like a string of beads this being due chiefly to 

 the alternate bands of brownish-white and black sur- 

 rounding the urns. In length the stems vary from one- 

 eighth to one -sixth of an inch, rarely reaching one- 

 fourth. To be seen on a wet stone with the unaided 

 vision, therefore, demands a trained eye. 



The cells or urns are joined end to end, the enlarged 

 central portion of each being light-colored, while both 

 the narrowed ends are dark or black. A single colony 

 is seldom formed of more than a dozen urns, the stems 

 thus built up being either quite straight or somewhat 

 curved, or even on occasion loosely coiled. At times 

 the stem is branched, the secondary stem originating 

 near the point of attachment of one cell with the pre- 

 ceding, but soon falling off or voluntarily breaking 

 away. On each side of every segment of the mature 

 stems is a small, cup-shaped projection, the two appear- 

 ing almost like handles to the urns. These are sup- 

 posed to be the remains of the branches or of those 

 segments which have fallen away and gone to begin 

 new colonies in another place. Each urn, therefore, has 

 at some time two urns attached to it, one on each side, 

 and occasionally a specimen will be found with one or 

 more branches still adherent. 



The central enlarged portion of the urns is translu- 

 cent, light-colored, and often with many transverse wrin- 

 kles and transverse brown lines. It is. also brown spot- 

 ted, and has many little tubercles of the same color. 

 The necks of the urns where they are joined together 



