DE. T. DAVIDSON ON RECENT BRACHIOPODA. 213 



their orij^in ; and form two spiral coils, with six or seven turns eat-h, directed inwards 

 and upwards. The arms are very thick at the base, taper more suddenly than in the 

 articulated species, and terminate in fine points. On making a transverse section they 

 are found to differ in organization from those of Uhjnchonellu, to which, externally, they 

 bear the strongest resemblance. Instead of one, as in that genus, there are two great 

 brachial canals, which may be denominated respectively the anterior and posterior. The 

 former is the equivalent of the great canal in the other Brachiopods, and, like it, 

 terminates at the side of the oesophagus in a blind sac. It is j)retty regularly 

 cylindrical, with the walls excessively thick, being composed, for the most part, of a white 

 cartilage-like su.bstance, which is most developed towards the side opposite the cirri. 

 These latter organs are supported upon a semicartilaginous grooved ridge, which is very 

 similar to that in Wuldhehnia, and in like manner supplied with a very ample brachial 

 fold. Muscular fibres are also provided for the movements of these parts ; but the 

 fibres, though similarly arranged, arc not so extensively developed. This canal is lined 

 with a muscular stratum, the fibres of which run in a transverse direction in the vicinity 

 of the brachial fold ; elsewhere they arc longitudinal or slightly diagonal. 



" The posterior canal is much flattened, and stretches along the inner surface of the arm, 

 extending from the base of the cirri halfway round it. The canal has the appearance 

 of being formed by a process of the pallial membrane similar to that which extends along 

 the lateral portions of the loop in Waldheimia. The posterior canals of the two arms 

 terminate at the sides of the oesopliagus behind the anterior canals, and are separated 

 from each other by a considerable space, which is divided on the median line by a delicate, 

 membranous septum. The two chambers thus formed communicate with the peri- 

 visceral cavity by two smaU oval orifices placed one on each side of the septum ; these 

 orifices open into the cavity close behind the oesophagus, and directly above the transverse 



fold of the anterior wall of the body It seems clear enough that the arms of 



Lingula have the power of extension in a greater or less degree (p. 811) The 



anterior canals in Lingula will perform the same ofiice as the great canals do in other 

 Brachiopods ; that is, they wiU give firmness to the parts, so that the cirri and the brachial 

 fold may be brought into play. When the arms are retracted the walls of these canals, 

 which it will be remembered are closed tubes, will relax a little to relieve themselves 

 from the pressure of the contained fluid ; when they are extended the walls will contract 

 to maintain the required pressure. Thus the cirri and brachial fold will be under the 

 control of their proper muscles, however much or little the arms may be extended. 



" The alimentary tube in Lingula (p. 815, loc. cit.) 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 occlnsors becomes compressed. The stomach is sliort, 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 adjuster muscles, gradually 

 descends towards the ventral lobe, immediatelv in front of the divaricator muscle. This 



