What is it? Is it a fungus or a plant parasite?

Please can any plant expert identify this organism on the twigs of a tree?

They are shown as dark brown objects in the picture (see link below)

There are hundreds of them. They are very small, no bigger than 0.5 cm or 1/5 inch each.

They are immobile (I've never seen them move!)

They seem to have some sort of a shell.

Ants are attracted to them.

Link to image on Flickr:
https://www.flickr.com/photos/peterpierre/16699506037/

Thanks

 

Appears to be a type of scale insect. The ants farm them and feed off of the honeydew they excrete.

Scale Management Guidelines--UC IPM​