CHAPTEE XII 



BEACHIOPODA 



Brachioi)oda-f ^•'^'''iiii^s — lAngula, Crania, Discina. 



^ \ Testicardines — Argiope, Terebratula, Waldkeimia. 



Chaeacteristics. — Coelomata devoid of organs of locomotion, and 

 usually fixed in the sand on to some foreign iody, hy a 

 peduncle. A Mvalved shell encloses the body. The valves 

 are dorsal and ventral, and in one subdivision are hinged to 

 one another. They are lined hy dorsal and ventral extensions 

 of the body-wall, termed the mantles ; these often hear chitin- 

 oid setae round their edges. A lophophore surrounds the 

 mouth, bearing ciliated tentacles. The alimsntary canal is 

 ciliated, and receives the secretion of two branched glands, the 

 liver ; it is in one sub-division aproctous. One, rarely two, 

 pair of nephridia exist. Uxclusively marine. 

 The existing BracHopoda are interesting as the survival of 

 what in early geological time was a 

 very widely distributed and very numer- 

 ous group of animals. The two genera 

 Lingula and Discina extend from the 

 Cambrian, the oldest group of the 

 Silurian rocks, to the present day ; and, 

 judging by their shells, they appear to 

 have undergone but httle change during 

 the vast period of time which must 

 have elapsed since they lived. They are 

 found in great numbers, both of indi- 

 viduals and of species, in these older 

 Paleozoic formations ; but the group seems to have been most 

 flourishing in the Devonian seas, for upwards of 60 genera and 



Fig. 106. — Waldheimia 

 cranium. 



A. Ventral, 



B. Dorsal valve. 



