246 



LOWER INVERTEBRATES. 



like that structure, support a greater or less number of tentacles, like a fringe. These 

 arras are long, and, in order to be accommodated within the shell they are folded, or 

 coiled in a spiral. In some species they can be slightly protruded from the shell, but 

 the extent of motion, in most forms, is small, since they are frequently supported upon 

 a calcareous process of the shell itself, a structure frequently preserved in fossil 

 forms. The Brachiopoda are divided into two groups, accordingly as the two valves 

 of the shell are hinged or not. 



Fig. 250. — Lingula pyramidata. 



Order I. — INARTICULATA. 



In the Inarticulata, or Ecardinia as it is sometimes called, the hinge of the shell is 

 wanting, as is also any calcareous support to the arms. The alimentary canal is com- 

 plete, the anus emptying into one side of the chamber of the mantle. The borders 

 of the mantle are completely separate. 



The family Lingulid^ embraces forms which have lived on the earth since almost 

 the earliest geological times. The genus Lingula appeared in the rocks of the Pots- 

 dam group, at the very base of the lower silurian, and to-day species of Lingula are 

 found in various warmer seas. In these the thin, horny valves of the shell are nearly 



equal and similar, while from near the 

 point of attachment of the valves pro- 

 ceeds a long, fleshy stalk, or peduncle. 

 The best-known species of Lingula to-day 

 is L. pyramidata, occurring on the sandy 

 shores of Virginia and North Carolina. 

 In this form the stem is about two inches 

 in length. The animal lives with its 

 peduncle buried in the sand, in water 

 of from one to ten fathoms, while the 

 shells, in the centre of which is the 

 mouth, project above the bottom. Not only is the. genus a long-lived one, but the 

 individuals them-selves are able to withstand very adverse circumstances. Speci- 

 mens can readily be carried to all parts of the country, and Professor Morse relates 

 that individuals which he obtained survived after being several hours loose in his 

 pocket. While our species of Lingula is small, those found in the eastern seas reach 

 a length of nearly a foot. The development of our species has been studied, and Dr. 

 Brooks says, " that the recent and fossil shells of the various species of Crania, I>is- 

 cina, lAngula, Lingulella, Obolus, and other hingeless brachiopods, furnish a series of 

 adult forms representing all the changes through which the 

 outline of the shell of Lingula pyramidata passed during 

 its development." Of Lingula there are now seventeen 

 living species, and a large number fossil. All the other 

 genera of the family, Obolus, Lingulella, etc., are extinct. 



The DisciNiDiE, in which the shell is nearly circular 

 and the peduncle passes through the flat lower valve, have 

 only a single existing genus, Liscina. These forms ex- 

 ternally closely resemble the genus Anomia, a true mollusc. 

 The CEANiiDiB, represented in the seas of Europe by the 

 genus Crania, have no peduncle. Fourteen species of the two families are found in 

 the existing seas. 



Fig. 251. — Upper valve of Crania 

 anomala, with the animal. 



