358 MANTJAL OF THE MOLLUSCi, 



food is obtained by means of cru'rents created by cilia, 

 Lingida may have the power of slightly extending the arms. 

 The internal skeleton consists of two spiral processes in the 

 Spiriferidce (Fig. 168), whilst in Terehratula and Thecidium it 

 takes the form of a loojj, which sup23orts the brachial mem- 

 brane, but does not strictly follow the course of the arms. The 

 mode in which the arms are folded is highly characteristic of 

 the genera of Bracliiopoda ; the extent to which they are sup- 

 ported by a calcareous skeleton is of less importance, and 

 liable to be modified by age. That margin of the oral arms 

 which answers to the lower lij) of an ordinary biyalve, is 

 fringed with long filaments {cirri), as may be seen even in dry 

 specimens of recent Terehratulce. In some fossil examples the 

 cirri themselves were supported by slender processes of shell ; * 

 they cannot, therefore, be vibratile organs, but are probably 

 themselves covered with microscopic cilia, like the oral ten- 

 tacles of the ascidian j)olypes {cilio-hrachiata of Farre). The 

 anterior lip and inner margin of the oral arms is plain, and 

 forms a narrow gutter along which the particles collected by 

 the cUiary currents may be conveyed to the mouth. The object 

 of the folding of the arms is obviously to give increased surface 

 for the disjDOsition of the cirri. 



The mouth conducts by a narrow cesojDhagus to a simple 

 stomach, which is surrounded by the large and granulated 

 liver; the intestine of Lingula is reflected dorsally, slightly 

 convoluted, and terminates between the mantle lobes on the 

 right side (Fig. 202). In Orhicida it is reflected ventrally, and 

 passes straight to the right, ending as in Lingida. In Terebra- 

 tida, Rhynclionella, and probably all the articulated Brachiopoda, 

 the intestine is simple and reflected ventrally, passing through 

 a notch or foramen in the hinge-j)late, and ending behind the 

 ventral insertion of the adductor muscle (Fig. 145, v.)t 



The circulatory system is far less complex than was formerly 

 supposed, and does not diflPer greatly from the same system in 

 the Tunicata. The heart is placed on the dorsal surface of the 

 stomach, and consists of a simple, unilocular, pyriform vesicle 

 without any auricle. From it the blood is propelled through 



* Spirifera rostrata and TerehraUda pectunculoides, in the British Museum. 



t The position at which the intestine terminates in the Terehratulce and Bhyn- 

 chonellcc, seems to necessitate the escape of tlie faeces by the mnbonal opening ; in 

 these extinct p;enera which have the foramen closed at an early age, there is still an 

 opening between the valves (<?. cj. in Uncites) which has been mistaken for a bj'ssul 

 notch. IVIi'. Hancock has carefully dissected several species of these genera without 

 detecting any anal aperture. Filling the intestines with injections was tried, but no 

 outlet could be discovered. 



