Lingula pyramidata (after Morse).
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.
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.
The stalk is an extension of the ventral body-wall, and contains a portion of the coelom which, in Discinisca and Lingula, remains in communication with the general body cavity.
In the Ecardines, of which Lingula and Discina may be quoted as examples, the myology is much more complicated.
In the Testicardines, where no such sliding action of the valves was necessary or possible, no muscles for such an object were required, consequently none took rise from the lateral portions of the valves as in Lingula; but in an extinct group, the Trimerellidae, which seems to be somewhat intermediate in character between the Ecardines and Testicardines, have been found certain scars, which appear to have been produced by rudimentary lateral muscles, but it is doubtful (considering the shells are furnished with teeth, though but rudely developed) whether such muscles enabled the valves, as in Lingula, to move forward and backward upon each other.
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.
With the exception of Yatsu's article on the development of Lingula (J.
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.
Frequently a temporary return of marine conditions permitted the entombment of such salt water genera as Lingula, Orbiculoides, Productus in the thin beds known as " marine bands."
A leathern tongue (lingula) is often seen to project from beneath the straps..
Owen not only occupied himself with the dissection of rare animals, such as the Pearly Nautilus, Lingula, Limulus, Protopterus, Apteryx, &c., and with the description and reconstruction of extinct reptiles, birds and mammals - following the Cuvierian tradition - but gave precision and currency to the morphological doctrines which had taken their rise in the beginning of the century by the introduction of two terms, " homology " and " analogy," which were defined so as to express two different kinds of agreement in animal structures, which, owing to the want of such " counters of thought," had been hitherto continually confused.
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.