12 INTRODUCTION. 



towards the aperture of the shell, and which part, from the downward bend of the mouth, 

 is made anterior, instead of being, as in the normal position of the mouth, dorsal. These 

 filaments are lost in the beginning of the spiral connecting fold, w, y, the brachial spire 

 thus receiving some of their nerves from the same part of the oesophageal ring, as the 

 antennae do in insects, and the cephalic tentacula in the higher MoUusks. The two chief 

 nerve-trunks from the ring, "i, figs, i and ii, come ofi" from its lower and lateral angles, a very 

 slight swelling, hardly to be called a ganglion, occurring at their origin. Each of these 

 trunks quickly divides, one division, t, going to the mantle, the other, tt, penetrating the 

 base of the fringed arm of its own side. The pallial trunk is the largest ; it soon divides 

 to supply the upper and lower mantle- lobes of its own side : the course of that distributed 

 upon the dorsal lobe is shown in pi. 2, fig. ii. 



■ The dorso-pallial nerve, /u, extends a short way upon it, and then expands into what 

 appears to be an oblong narrow ganglion ; but which is a loop formed by the slight 

 divarication and reunion of the fibres of the trunk. From this loop, v, most of the pallial 

 filaments diverge. In their course towards the margin of the lobe they cross obliquely the 

 great pallial sinus, and give oft" branches, 1/ most of which correspond with the branches of 

 the sinus, these branches subdividing, and their ramifications appearing to unite in a 

 common circumpallial nerve, o, which runs along the inserted bases of the marginal 

 cilia. The brachial nerves, tt, fig. 1, may be traced some way along the muscular canal 

 of the fringed arms. 



Two delicate filaments which traverse part of the visceral chamber come off" from 

 the lower part of the oesophageal circle near the origins of the great pallio-brachial trunks ; 

 they probably supply the muscles which traverse the visceral chamber, as well as the hearts 

 and ahmentary canal ; but the indications of this part of the system were too faint and 

 uncertain, in Terebratula, for safe depiction. 



In Lingula they are more distinct, and are shown in fig. iii. The nerves, marked p, 

 come off from the suboesophageal ganglion, diverge as they pass backwards along the 

 visceral chamber, then converge to their insertion in the anterior muscles. The nerves, <t, 

 come off" directly from the suboesophageal ganglions, and run more parallel as they pass 

 along the ventral aspect of the anterior muscles to go to the posterior muscles. More delicate 

 filaments pass to the alimentary canal and the hearts. Lingula has also the pallial and 

 brachial systems of nerves as well developed as in Terebratula. I have not been able to 

 detect any traces of special sense-organs in the Brachiopoda. 



DIGESTIVE SYSTEM. 



The mouth, pi. 1, figs. 1 and 4, t, of the Terebratula, opening downwards, as before 

 remarked, at the base or beginning of the passage formed by the spiral brachial fold and 

 the converging fringes of the brachial spire, has a tumid and sub-bilobed upper or 

 anterior hp, and a thinner and broader lower or posterior hp, which is attached to the 



