422 BRACHIOPODA ; TEREBRATULA ; ORBICULA ; CRANIA. 



be ranged under another genus) they are enormously developed ; 

 and being quite free except at their origin, they may be extended 

 far beyond the shell. When drawn in, they are disposed in six 

 or seven spiral turns. The mechanism by which these arms are 

 unfolded is very curious. The stem of each is tubular, and 

 contains a fluid ; which, being acted upon by a multitude of 

 muscles forming the walls of the canal, is forced onwards up the 

 tube, and thus causes the arm to project. We shall hereafter 

 meet with a very similar contrivance, in the numerous tubular 

 feet of the Echinodermata (1007). The Terebratulae are 

 found in all seas, from the polar to the equinoctial, at a depth 

 of from ten to ninety fathoms, or even more. 



967. The Orbicula may be noticed as the type of the sessile 

 group. Its two valves differ considerably in form and size ; 

 one being conical and rounded, like the shell of the Limpet 

 (with which it was formerly confounded), whilst the other is flat, 

 with a fissure in the centre for the passage of a ligament, by 

 means of which it is fixed to the rocks. The Mollusk has 

 ciliated arms, rolled up spirally when withdrawn. The recent 

 species of this genus are found attached to stones, shells, and 

 sunken wrecks, at various depths, down to seventeen fathoms. 

 They sometimes are found in vast numbers in particular spots. 

 The Crania, another sessile genus, has been dredged up in 255 

 fathoms' water. These facts are of much interest, when taken in 

 connection with the circumstances, under which the fossil species 

 are found. It may be inferred, from the diminished numbers of 

 this class, that the ocean is at present less fit for their habitation 

 than it formerly was ; and perhaps a diminution of depth is one 

 of the circumstances which has given rise to this change. 



