They form, however, at the base of the peduncle distinct bundles (adjustor muscles of Hancock) which are inserted on the valves of the shell where they cause impressions.
1 The preceding description of the lophophore in the text is based on a comparison of recent memoirs and such specimens as were accessible with Hancock's figures and descriptions. Compare the following figures in his Memoir, Ph. Tr. 148, 1858; PI. LV, fig. I, Waldheimia australis; fig. 2, Terebratula caput serpentis; fig. 3, Rhynchonella; Pl. LVI, figs. 2 and 3, Waldheimia australis; PI. LXV, figs. 6, 7, 8, Lingula.
The central nervous system consists of a peri-oesophageal ring of some size, its ventral part usually lying posterior to the cirri, its dorsal anterior to the lip. In Lingula pyramidata this ring contains five ganglionic enlargements, two dorso-lateral, or supra-oesophageal, two ventro-lateral, and a median ventral, or sub-oesophageal. The ganglia lie immediately beneath the ectoderm, which here consists of one or more layers of cells supported on their inner aspect by connective tissue. The ring of Crania is said to contain one dorsal, or supra-oesophageal, ganglion, and two sub-oesophageal. As to the Testicardines, Hancock has described in Wald-heimia australis a ring similarly constituted to that of Lingula (supra), but van Bemmelen finds only a single supra- and a single sub-oesophageal ganglion connected by a commissure. So too in Argiope Kowalewskii according to Shipley, whereas Schulgin found in the same animal three sub-oesophageal ganglia, i. e. two ventro-lateral and a median. All the ganglia are in continuity with the ectoderm in Argiope (Shipley), but van Bemmelen found this to be the case only with the sub-oesophageal. Ganglion cells occur in the commissures, as well as in the nerves to the lophophore, or arms.
Three nerves pass, according to van Bemmelen, along each arm, one from both the supra- and sub-oesophageal ganglion, the third from the commissure, and are connected, especially that from the supra-oesophageal ganglion, with a rich ganglionic plexus. The sub-oesophageal ganglion gives off nerves to both the dorsal and ventral mantle-folds, in which they branch as they pass towards their free margins, as well as to the muscles and viscera. The disposition of the nerves is similar in Lingula (Beyer). Special sense organs are usually said to be wanting in the adult. Rudimentary eyes are stated by Schulgin, citing Kowalewsky, to be found in Megerlia 'on the inferior,' i. e. posterior 'margin of the tentacular disc not far from the mouth.' In Argiope Kowalewskii two parallel bands of massed ectoderm cells lie, according to Schulgin, posterior to the lophophore not far from the mouth, the one nearest consisting of 'specific,' i. e. sensory cells, innervated from the sub-oesophageal ganglia. The same author states that rod-like bi-nucleate sense-cells, connected basally to a nerve-filament, are found among the pyramidal ectoderm cells of the margins of the mantle.
Joubin has traced in Crania filaments from the brachial nerve plexus to pads of elongated ectodermic cells at the bases on the inner aspect, i. e. the one turned to the brachial groove, of each cirrus, and to a continuous band of similar cells on the outer aspect of the same structures.
1 They do not occur in Lingula anatina, .L. affinis, Waldheimia or Rhynchonella.
The mouth opens into the brachial groove at a spot coinciding with the centre of the convexity of the horseshoe. It leads into an oesophagus which turns anteriorly and dorsally, and then curves back upon itself and passes into the stomach. The latter is enlarged, and receives the ducts of the so-called liver lobes, which vary in number in different Brachiopoda, e.g. two in Argiope, etc, five in Lingula pyramidata. The stomach is followed by the intestine, which turns towards the ventral valve in the Testicardines, and, sooner or later, ends blindly. The Ecardines have an anus. In Lingula pyramidata the intestine skirts along the right side of the body; but in L. anatina it forms two loops before it does so; in Dis-cina the curvatures are more complex; in Crania more simple, consisting of a three-quarter circle to the left, and in both there is a terminal dilated rectum. This rectum in Crania lies in the middle line between the two posterior adductor muscles, and the anus opens into a sinus between the posterior ends of the two valves, where the hinge and peduncle should be 1, and not into the pallial cavity between the two valves as in Lingula and Discina, where it lies on the right side but nearer to the ventral valve than the dorsal.
Whether the anus has or has not become aborted in the Tes-ticardines is a difficult question. Probability is. of course, in favour of the former opinion. The walls of the digestive tract consist of an outer layer of supporting substance lined by ciliated epithelium1. A 'liver' lobe in Argiope consists of 6-j simple elongated caeca opening by a simple common duct. The ducts are, however, usually ramified forming lobules. The caeca are lined by a non-ciliated glandular epithelium. The food of Brachiopoda consists of Diatoms, unicellular Algae, etc, collected by the ciliated epithelium probably of both the cirri and brachial groove. Processes of the outer, or supporting, tissue coat of the digestive tract serve to suspend it in the body cavity. Their general disposition is as follows. A median dorsal lamina connects it to the dorsal shell, while a ventral lamina unites the oesophagus, stomach, and intestine, and is in Testicar-dines attached also to the ventral end of the occlusor muscles, or in Crania among Ecardines to the ventral valve.
These laminae constitute the 'mesentery' of Huxley. The commencement of the stomach is connected by two (Linguld) or three (some Testicardines) 'gastro-parietal' bands, absent in Crania, to the dorsal shell at the attachment of the posterior occlusor muscles, while a right and left 'ileo-parietal' band tie it to the side walls of the body behind the junction of the two mantle-folds. The perivisceral portion of the coelome is large, and is broken up by the viscera, muscles and supporting bands of the digestive tract. It is continued outwards into each lobe of the mantle as the pallial sinuses, which are usually two or four in number in each lobe. The sinuses branch, as a rule, in a manner characteristic of the genus or species, and in Lingula anatina those branches which run radially and side by side towards the free margins of the mantle lobes are close-set, thin-walled, and terminate in small ampullae.
1 See Joubin, A. Z. Expt. (2), iv. 1886, pp. 218-19, figs. 2, 3, and p. 233.