VALVES OF TEREBKATULA. 511 



its body ; or else, as they are most probably delicate organs of touch, 

 they may perform the office of highly sensitive tentacula. 



(1344.) In Terebratula psittacea the arms are enormously developed, 

 fringed upon their outer margins, and quite free except at their origins : 

 when completely contracted, they are disposed in six or seven spiral 

 folds ; and when unfolded, they extend beyond the shell twice its longi- 

 tudinal diameter. The mechanism by which they are unfolded is de- 

 scribed by Professor Owen* as being extremely simple and beautiful. 

 The principal stem of each arm is hollow from one end to the other, 

 and contains a fluid, which, being acted upon by the spirally-disposed 

 muscles forming the parietes of the canal, is forcibly injected towards 

 the extremity of the arm, and the organ is thus expanded and protruded 

 outwards. 



(1345.) In Terebratula ChUensis, on the contrary, the movements of 

 the arms are extremely limited, and they can no longer be protruded 

 from the shell as in the preceding species, being connected throughout 

 their whole length with a peculiar complex testaceous apparatus attached 

 to the internal surface of the imperforate valve of the shell (fig. 257, B), 

 the arrangement and uses of which are thus described in the memoir 

 above-mentioned : The principal part of the internal framework alluded 

 to consists of a slender flattened calcareous loop (//), the extremities 

 of which are attached to the lateral elevated ridges of the hinge : the 

 crura of the loop diverge, but again approximate each other as they 

 advance for a greater or less distance towards the opposite margin of 



9 

 A B 



Valves of Terebratula Chilensis, showing the internal framework. (After Owen.) 



the valve ; the loop then suddenly turns towards the imperforate valve, 

 and is bent back upon itself for a greater or less extent in different 

 species. The loop, besides being fixed by its origins, or crura, is com- 

 monly attached to two processes (d d) going off at right angles from the 

 sides, or formed by a bifurcation of the extremity of a central process 

 (c), which is continued forwards from the hinge; but it is sometimes 

 entirely free except at its origins. The arches of the loop are so slender, 



* Transactions of the Zoological Society, vol. i. 



