466 RECENT BRACHIOPODA 



CHAP. 



one on each side of the body, but occupy a different position 

 with regard to the main axes of the body. What this position 

 is, has formed the subject of a good deal ot discussion. For our 

 purpose, however, it will suffice to distinguish the two valves by 

 the most commonly accepted terms of dorsal and ventral. The 

 former is, as a rule, the smaller of the two, and usually lies 

 on the lower surface of the animal in life. Adopting the orien- 

 tation indicated above, the stalk by means of which the Brachio- 

 poda are attached to the rocks and stones, etc., upon which 

 they live, becomes posterior, and the broader edge of the two 

 shells, which are capable of being opened to some extent, is 

 anterior. 



The posterior end of the shell usually narrows, and the 

 ventral valve projects behind the dorsal, and may be produced 

 into a sort of beak or funnel, through the aperture of which the 



Fig. 312. —Four specimens of 

 Terehratulina caput serpen- 

 tis, attached to a water- 

 logged piece of wood, from 

 the Clyde area. 



stalk protrudes. This aperture may be completed by the ventral 

 shell, or the latter may only be notched, in Avhich case the hole 

 is completed by the posterior edge of the dorsal shell. 



The nature of the shell has been used in classifying the 

 group into two orders : — 



I. The Ecardines, whose shell is chitinous but slightly 



strengthened by a deposit of calcareous salts. There 

 is no hinge and no internal supports for the arms. 

 The alimentary canal terminates in an anus. 



II. The Testicardines, whose shells are composed of calca- 



reous spicules. The valves are hinged together, and 



there is usually an internal skeleton supporting the 



arms. There is no anus. 



The outside of the shell of recent Brachiopods is often smooth, 



but many are ridged. In a recent species, Rhynchonella Ddder- 



leini from Japan, Davidson^ has described a number of spines 



1 "On a living Spinose Bhynchonella from Japan," Ann. Mag. Nat. Hist., 

 6tli ser., vol. xvii., 1886, 



