BRACIIIOPODA. 491 



manner, so far unfolded as to react upon the closed valves of the 

 shell. 



What power the animal may possess of further unfolding and pro- 

 truding the free extremity of the united spiral portions of the arms, 

 can only be determined after careful observation of the living Tere- 

 hratulce in their native localities. The structure of the parts in 

 question led me to note, in 1833, the important difference between 

 Lingula and those species of Terehratula which resemble Ter. chi- 

 lensis in the structure of the arms, "since, from their attachments, 

 they are fixed and cannot be unfolded outwards as in Lingula.''* 



The muscular stem, by means of its attachment to the calcareous 

 loop (Jig- 187, a, b, c), 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 slightly from the 

 opposite one. Observations on ''living Mollusks are, however, essen- 

 tial to the formation of adequate and exact ideas of the uses of parts 

 of the several muscular parts of their organisation. 



The fringe-filaments of the produced and reflected portions of the. 

 brachia are in a single series ; they are compressed, very narrow, 

 close set, with their f]at sides to^vards each other, very gradually 

 tapering to the extremity which is slightly bent, the rest of the fila- 

 ment being usually straight : those(^^. 186, m)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 or channel. The curve of the spire is 

 towards the dorsal valve at its commencement ; and this, with the 

 dorsal aspect of the transverse base of the arms, serves to determine 

 the dorsal valve in Orbicula and Lingula. By Cuvier"|* the spiral 

 arms of the Brachiopods are compared with the tentacles of the 

 Cephalopods : by Siebold to those of Bryozoa (AlcyonellaX) ; perhaps 

 the true homology lies midway : the closer connection of the soft un- 

 calcified base of the arms to the mouth, in Lingula, seems transitional 

 to the long labial tentacles of Anomia. 



The nervous system of the Terebratula consists of three principal 

 parts, the " pallial," " brachial," and " visceral." The roots or origins 

 of these three systems centre in the oesophageal ring.§ 



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



* CCCI. vol. i. p. 149. t CCC. Mem. sur la Lingule, p. 4. 



X XXIV. p. 260. This idea is reproduced in CCCIX. p. 117. 

 ^ CCCIII. C, pl- ii. fig- 1. 



