THE BEACHIOPODS. 315 



is found in the Indian Ocean and the Pacific as far as the 

 coral zone extends. The animal 

 of the tridacna, and of the near- 

 ly related Hippopus, distinguishes 

 itself by the heauty of its colours. 

 The mantle of the Tridacna sa- 

 franea, for instance, has a dark 



blue edge with emerald-green ^^^^^^^^^^^^^^ 

 spots, gradually passing into a 

 light violet. When a large num- 

 ber of these beautiful creatures 

 expand the velvet brilliancy of Hippopua macuiatus. 



their costly robes in the transparent waters, no flower-bed on 

 earth can equal them in splendour. 



Like the Lamellibranchiate Acephala, the Brachiopods are 

 covered with a bivalve shell, but their internal organisation is 

 very different. Instead of being disposed in separate gills, 

 their respiratory system is combined with the ciliated mantle on 

 which the vascular ramifications are distributed, but their most 

 striking feature is the possession of spiral fringed arms or 

 buccal appendages which serve to open the shell and occupy 

 the greater part of its cavity. These curious organs are in some 

 Brachiopods quite free, in others attached to a complicated 

 cartilaginous or calcareous skeleton. None of the existing 

 molluscs of this class are capable of changing place, but are 

 either fixed to extraneous substances by the agglutination of one 

 of their valves or by a muscular peduncle passing through a 

 perforation of their shells. There are no more than forty-nine 

 living species, chiefly belonging to the genera Terebratula and 

 Crania, and generally found at great depths in the Southern 

 Ocean; but the fossil remains of 1,370 species prove then- 

 importance in the primitive seas, where they rivalled the 

 lamellibranchiates in numbers and variety. Though now so 

 rare or so local in the British seas that ordinary collectors are 

 not likely to meet with any, they abound in many of our oldest 

 rocks. " A visit to the quarries at Dudley," says E. Forbes, 

 " or an Irish lime-kiln, or an oolitic section on the Dorsetshire 

 coast, or a green sand ravine in the Isle of Wight, will afford 



