In Lingula and Discina the anus lies to the right in the mantle-cavity, but in Crania it opens medianly into a posterior extension of the same.
Undue stress is often laid on the fact that Lingula has come down to us apparently unchanged since Cambrian times, whilst Crania, and forms very closely resembling Discina and Rhynchonella, are found from the Ordovician strata onwards.
In Lingula the shell is composed of alternate layers of chitin and of phosphate of lime.
In Discinisca and Lingula, however, the sub-oesophageal ganglion is not drawn out, but lies medianly; it gives off two posteriorly directed nerves to the stalk, which in Lingula unite and form a substantial nerve.
In the Ecardines, of which Lingula and Discina may be quoted as examples, the myology is much more complicated.
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.
Below 150 fathoms they are rare, but a few such as Terebratulina wyvillei are found down to 2000 fathoms. Lingula is essentially a very shallow water form.
With the exception of Yatsu's article on the development of Lingula (J.
In Crania it is completely shut off from the main coelom, but in Lingula it communicates freely with this cavity.