278 LECTURE XX. 



hollow. In the Tei'ebratidce and Orhicul(B the spiral terminations of 

 the arms have their central canal surrounded by a double oblique 

 series of muscular fibres : the canal is filled with fluid, and, by the 

 contraction of the muscular fibres, the extremities are extended by 

 the pressure of the contained fluid which is injected into them. 



The alimentary canal is very short and simple : in the Terebratula 

 it soon expands into a gastric cavity, surrounded by groups of the 

 minute hepatic follicles, included in the peritoneal membrane behind 

 the base of the arms. The stomach bends, and is continued in a 

 short and straight intestine, which terminates between the lobes of 

 the mantle, on the right side. A small transversely plicated membra- 

 nous process is continued from each side of the beginning of the 

 intestine. 



The principal centres of the vascular system are two dilated sinuses 

 or ventricles, situated on each side of the visceral mass, and commu- 

 nicating by one extremity with the vessels which ramify over the 

 lobes of the mantle, and by the opposite extremity with those which 

 supply the viscera. In the Terebratula and Orhicula, the respiratory 

 function would seem to be performed chiefly by the pallial vessels; 

 and in this conclusion we are supported by observing the prolongation 

 of vascular loops from the pallial vessels in the Lingula, which 

 affords the only indication of distinct branchial organs in the class 

 Brachiopoda. The margin of the mantle is fringed with cilia, which 

 are very long in the Lingida and Orhicula. In the latter Brachiopod 

 I found them beset with smaller cilia: these, with the brachial 

 fringes, and the microscopic vibratile cilia, which, doubtless, beset 

 the whole surface of the vascular mantle, must be the chief agents in 

 introducing the currents of sea water within the cavity of the mantle 

 for nutrition, respiration, and excretion. 



A nervous collar, with small ganglionic enlargements, surrounds 

 the oesophagus in the Terebratida, as in the Lingida and Orbicula. 

 Nervous filaments are continued from this centre to the adductor 

 muscles, and to the vascular lobes of the mantle. 



In the Terebratula, two pairs of muscles arise from each valve, 

 some of which are attached to the opposite valve, and others lost in 

 the pedicle. The pedicle is surrounded by an elastic yellow horny 

 layer and a tubular prolongation of the mantle. In the Orbicula 

 there are eight distinct muscles adapted for closing the shell, sliding 

 the valves upon each other, and attaching them to foreign bodies. 



In the Terebratula and Orbicula the ovaria are dendritic, and 

 attached to the mantle, four on the lower and two on the upper lobe : 

 the ova appear to be developed from the inner surface of the large 

 and wide pallial veins, to the parietes of which they adhere. The 



