ANATOMY OF THE TEREBRATULA. 11 



observing with respect to the Ter. aiistralis, Ter. vitrea, and other species of his genus 

 Terehratula proper, that, " from its texture and relations it never could be moved or 

 unrolled at the will of the animal ;" and he adds, " the sole use of the folding of the arms 

 is to give increased surface for the disposition of the cilia." By the latter term, the author 

 means what I have here and elsewhere called the ' filaments,' forming the fringe. But 

 comparison of the parts in action of a living MoUusk, with the same parts in a dead 

 specimen stifiened in alcohol, has in most cases tended greatly to expand the observer's 

 notions of the powers and uses of such parts derived from dissections ; and it would be 

 rash to conclude that so well-organised and beautiful a mechanism for acting on the fringed 

 arms in the direction of their length, as is shown in figures ii, pi. 2, and ii, pi. 3, should 

 exist there without a purpose, as would be the case were the folding of the arms solely for 

 the purpose of augmenting the surface for the attachment of the filaments. In my earlier 

 Memoir, while appreciating and admitting the great diflerence in the arrangement of the 

 arms for the facilitating their movements of extension, in different species of TerehratiddB, 

 since raised to the rank of genera, other uses of the foldings of the loops had also suggested 

 themselves. " 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 sufficient 

 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 shghtly from the opposite one."' Obser- 

 vations on living MoUusks are, however, essential to the formation of adequate and exact 

 ideas of the uses of parts of the several muscular parts of their organisation. 



The fringe-filaments, y, §, pi. 3, figs, ii and iii, of the produced and reflected portions 

 of the brachia, are in a single series ; they are compressed, very narrow, close set, with 

 their flat sides towards each other, very gradually tapering to the extremity which is 

 slightly bent, the rest of the filament being usually straight : those, i, of the spirally- 

 disposed portion are split at the end, and the split is deeper as the filaments are situated 

 nearer the end of the spire, where they appear thereby to be arranged in a double row. 

 In most specimens, the filaments of the spire incline towards each other, and meet at their 

 extremities, inclosing a triangular space, as shown in pi. 3, fig. ii, in which the end of the 

 spire has been cut ofi". 



NERVOUS SYSTEM. 



The nervous system of the Terebratula consists of three principal parts or systems, the 

 "pallial," "brachial," and "visceral." The roots or origins of these three systems centre 

 in the oesophageal ring, ^, pi. 2, fig. i. 



This annular centre is situated in and defended by the basal fold of the brachial 

 aponeurosis, ib. iv, surrounding the aperture by which the oesophagus penetrates the visceral 

 chamber. A few very delicate filaments pass off from the part of the ring which is turned 



1 Tom. cit., p. 150. 



