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.
These spaces are as follows: - (i.) the great arm-sinus; (ii.) the small arm-sinus together with the central sinus and the peri-oesophageal sinus, and in Discinisca and Lingula, and, to a less extent, in Crania, the lip-sinus; (iii.) certain portions of the general body cavity which in Crania are separated off and contain muscles, &c.; (iv.) the cavity of the stalk when such exists.
In Crania it is completely shut off from the main coelom, but in Lingula it communicates freely with this cavity.