250 AQUATIC MICROSCOPY FOR BEGINNERS. 



colonies, or stem-like growths which it forms, are 

 composed of urn-shaped cells or segments united end 

 to end, and attached by a single disk-like enlargement 

 to the supporting object from which, they hang sus- 

 pended. The lower surface of stones, beneath which 

 the water constantly flows, seems to be Urnatella's 

 favorite haunt. The stem-like colonies of urns are 

 usually found two together pendent from the same disk 

 of attachment, and appearing somewhat like a string 

 of beads — this being due chiefly to the alternate bands 

 of brownish-white and black surrounding each urn. 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 en- 

 larged 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 straight or somewhat 

 curved, or even, on occasion, loosely coiled. At times 

 the stem is branched, the secondary limb 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 appearing 

 almost like handles to the urns. These are supposed 

 to be the remains of 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. 



