In Lingula the shell is composed of alternate layers of chitin and of phosphate of lime.
In Terebratulina, Rhyn- chonella, Lingula, and possibly other genera, the arms can be unrolled and protruded from the opened shell; in this case the tentacles also FIG.
In Crania it is completely shut off from the main coelom, but in Lingula it communicates freely with this cavity.
In Discinisca and Lingula there is further a lip-sinus or hollow system of channels which traverses the supporting tissue of the edge of the mantle and contains muscle-fibres.
It is better developed and more spacious in Lingula than in Discinisca.