492 LECTERE XX. 



of the brachial aponeurosis, 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 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 be- 

 ginning of the spiral connecting fold, the brachial arms 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 come off 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 going to the mantle, the other pene- 

 trating 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 Plate 11. Jig. 2 of CCCIII. 



The dorso-pallial nerve 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 most of the pallial filaments 

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

 obliquely the great pallial sinus, and give off branches, most of 

 which correspond with the branches of the sinus, these branches 

 subdividing, and their ramifications appearing to unite in a common 

 circurapallial nerve which runs along the inserted bases of the mar- 

 ginal cilia. The brachial nerves 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 alimentary canal. In Lingula 

 the visceral nerves are more developed. They are shown mjig. 3, 

 PI. II., CCCIII. Filaments to the muscles are also more distinct : a 

 pair, which come oflf from the subccsophageal ganglion, diverge as they 

 pass backwards along the visceral chamber, then converge to their 

 insertion in the anterior muscles ; a second pair, also from the sub- 

 oesophageal ganglions, run more parallel as they pass along the 

 ventral aspect of the anterior muscles to go to the posterior muscles. 

 Lingula has also the pallial and brachial systems of nerves as well 

 developed as in Terehratula. I have not been able to detect any 

 traces of special sense-organs in the Brachiopods. 



