THE BRITISH FOSSIL BRACHIOPODA. 335 



longer than in Waldheimia, the mouth approaching much nearer to the ventral valve. 

 The intestine is rather long and gradually tapers downwards. On reaching the ventral 

 valve, directly behind the extremities of the occlusor muscles, it turns backwards and 

 upwards, and, detaching itself from the mesentery, advances a little and terminates in a 

 much enlarged, rounded extremity, which inclines to the right or left, varying in this 

 respect in different individuals. 



The alimentary tube in Lingula, says Hancock at p. 815 of his work already referred 

 to, " presents two or three interesting modifications. The mouth is perfectly similar to 

 that of Waldheimia, and the oesophagus, which is somewhat elongated, is at first 

 depressed ; but on emerging backwards from between the anterior occlusors, becomes 

 compressed. The stomach is short, being almost lenticular, and the transverse dorsal 

 ridge is much produced. In front it is slightly convex, where it receives the 

 oesophagus ; behind it is a little produced, and an inconspicuous constriction marks 

 the commencement of the intestine, which running backwards, buried beneath the 

 lobules of the liver, and resting above the posterior adjustor muscles, gradually 

 descends towards the ventral lobe, immediately in front of the divaricator muscles. 

 The straight portion of the intestine corresponds to the entire intestine of the Arti- 

 culated Brachiopoda, and it thus appears that in Lingula, as well as in them, the first 

 inflection of the intestinal tube is towards the ventral surface. From thence the tube 

 bends to the left, and, turning forwards and upwards, forms two large loops at the 

 posterior portion of the perivisceral chamber ; it then advances along the right side, and 

 dipping under the dorsal extremities of the adjustor muscles and the pseudo-heart, 

 opens through a nipple-like anus, situated at the right side of the body between the 

 margins of the mantle. The anal orifice is ample, and is very easily distinguished ; it is 

 placed considerably nearer to the dorsal than the ventral surface. The calibre of the 

 alimentary canal does not vary much ; the stomach only being a little enlarged, &c." 



The nervous system has been likewise elaborately described by Owen, Hancock, 

 Bemmelen, and others, and its condition of development is lower than that of the Lamel- 

 libranchs. Hancock again says (p. 845) that the collar in Waldheimia " is situated at the 

 commencement of the alimentary tube, and there are five nervous centres, three of which, 

 on account of their superior size, may be assumed to be the principal oesophageal ganglia." 

 Van Bemmelen states (see 'Ann. and Mag. of Nat. Hist.,' 5th series, vol. xi, p. 381, 

 1883) that the central nervous system consists of a circum oesophageal ring, which not 

 only shows a large infracesophageal but also a supracesophageal ganglion. Both these 

 centres give origin to arm-nerves ; the nerves of the supraoesophageal are even more 

 important than those of the other ganglion. The nervous centra are formed by thin 

 nervous fibres and very small cells, the peripheral nerves by parallel fibres only, between 

 which nuclei of connective tissue, or perhaps of nervous elements, are to be found. 

 The nerve-arms are surrounded by and connected with a ganglionic plexus, situated in 

 the supporting tissues of the arm-walls immediately under the ectodermal epithelium, 



