212 MANUAL OF THE MOLLUSCA. 



narrow gutter along which the particles collected by the ciliary currents ma> 

 be conveyed to the mouth. The object of the folding of tBfe arms is obvi- 

 ously to give increased surface for the disposition of the cirri. 



The month conducts by a narrow oesophagus 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. 165). In Orbicula it is reflected ventrally, and 

 passes straight to the right, ending as in Lingula. In Terebratula, Rhyncho- 

 nella, and probably all the normal Brachiopoda, the intestine is simple and 

 reflected ventrally, passing through a notch or foramen in the hinge-plate, 

 and ending behind the ventral insertion of the adductor muscle (fig. 109, v.) * 

 The interior of the valves is lined by the two lobes of the mantle, which 

 are often fringed with fine horny bristles (seta] ; these are quite straight, 

 brittle, and deeply implanted between the laminae of the mantle ; they serve 

 to guard the opening of the valves. The mantle-lobes of the Brachiopoda 

 are not only organs by which the shell is formed, they are also provided with 

 large veins by which respiration is effected ; in the Terebratulidte there are 

 two great venous trunks in the dorsal mantle- lobe, four in the ventral ; in 

 Rhynchonella and Dlscina the lobes are similar, and the Orthidce have four 

 large veins in the dorsal lobe and only two in the ventral. The first indica- 

 tion of a special breathing organ is presented by Lingula, in which the veins 

 develope parallel rows of small vascular processes. (Cuvier.) The veins 

 open into the visceral cavity, f which is itself a great vascular sinus. There 

 are two organs which Prof. Owen regards as hearts, each consisting of an 

 auricle and a ventricle, situated near the sides of the mouth in Terebratula ; 

 but in Lingula (fig. 165, h.} they are more posterior, and quite at the sides. 

 The ventricles propel the blood into the visceral and pallial arteries, and are 

 therefore both branchial and systemic. The pallial arteries are very slender, 

 and accompany the veins on their outer surfaces, forming linear impressions 

 along the centre of the vascular markings in some fossil shells (fig. 141). 



The ova of Terebratula are developed within the large veins, which they 

 accompany as far as the secondary branches. In the Rhynchonellida, anc 

 probably in the extinct Orthidte, the ovaria do not extend into the venous 

 trunks, but occupy large sinuses on each side of the body ; and in Discina 

 and Lingula they (or the testes) fill the interstices of all the viscera, but do 

 not appear to extend into the mantle. The ova are supposed to escape by 

 two orifices, situated at the sides of the mouth in Terebratula. (Hancock.) 



* The position at which the intestine terminates in the Terebratulce and Rhyn- 

 chonellce, seems to necessitate the escape of the farces by the umbonal opening ; in 

 those extinct genera which have the foramen closed at an early age, there is still an 

 opening between the valves (e. g. in Uncites) which has been mistaken for a byssal 

 notch. 



t The veins do not terminate in hearts as formerly supposed \. the statement at 

 p. 30, line 27, should be erased. 



