150 MR. R. OWEN ON THE ANATOMY OF THE BRACHIOPODA. 
The arms in Ter. Chilensis, when detached from the supporting processes and un- 
folded, exceed the length of the shell by two thirds of that length; and their length 
is to their breadth as eight to one. Their stem, from which the cilia arise, as it has 
not to execute the movement of JLingula, so it is much more slender. The cilia, 
therefore, are proportionately increased, in order to excite the necessary currents in 
the water; which, being directed between the folds of the arms towards the mouth, 
as to a focus, carry thither the nutrient molecules, which are retained by the natural 
sieve formed by the decussating cilia of the terminal processes in front of the mouth ; 
and though this apparatus be apparently less perfect than in Lingula, it is evidently 
adjusted in due relation to the support of so small a mass of body as exists in Tere- 
bratula. The muscular stem, by means of its attachment to the calcareous loop, has 
the power of acting upon that part to the extent its elasticity admits of, which is suffi- 
cient to produce such a degree of convexity in the reflected part of the loop, as to cause 
it to press upon the perforated valve, and separate it slightly from the opposite one. 
This elastic internal apparatus thus compensates for the absence of the thick protruding 
arms which push open the valves in Lingula, and for the want of the elastic fibres which 
constitute the ligament of the hinge in ordinary Bivalves ; and it is apparently the only 
means this and similar species of Terebratula possess of divaricating the valves. 
In other species in which the loop is wanting, as in Ter. rubicunda, Sow., there is a 
compensating structure; the furcated extremity of the central calcareous process is 
developed to a great size, and the forks can be approximated and separated from each 
other to a small extent, to effect the same purpose as the loop in the normal Terebratule. 
In Ter. vitrea, however, the loop, though perfect, is too small to be capable of being 
made to press upon the perforate valve in the way in which I suppose it to act in the 
more depressed species, as Ter. dorsata, Ter. dentata, Ter. Sowerbii, and Ter. Chilensis, 
in all of which it is largely developed for that purpose. It is probable, therefore, that 
the arms in this species have a different disposition, and possess greater powers of ex- 
tension to compensate for the small development of the internal skeleton. 
It is by such a modification of the structure of the soft parts that the opening of the 
shell is effected in Ter. psittacea. ‘The internal skeleton in this species is reduced to 
two small processes, curved slightly outwards, which are continued from the sides of 
the hinge of the imperforate valve. From these processes arise two spiral arms', fringed 
on their outer margin, as in the other species, but quite free, excepting at their origins. 
When contracted they are disposed in six or seven spiral gyrations, decreasing towards 
their extremities ; and when completely unfolded they extend beyond the shell twice 
its longitudinal diameter. The cilia are more curved than in Ter. Chilensis, and the 
stem which supports them is more muscular. The mechanism by which the arms are 
extended is simple and beautiful: the stems are hollow from one end to the other, and 
are filled with fluid, which, being acted upon by the spirally disposed muscles com- 
1 Fig. 14*, 
