CLASSIFICATION OF THE BRACHIOPODA. 



127 



Fig. 47. 



Fig. 48. 



Fig. 49. 



t'm 



sill 



Discina lamellosa, Brod., Peru, Rev. W. Hennah.' 



Fig. 4 7. Animal, as seen on the removal of the lower valve, and part of (he ventral mantle-lobe ; the extremities of the labial 

 arms are displaced forwards, in order to shoiv their spiral terminations ; p, is the expanded surface of the plug or 

 pedicle ; the mouth is concealed by the overhanging fringe. 



Fig. 48. Dorsal mantle-lobe with its fringe of seire. 



Fig. 49. Ventral mantle-lobe {the sette not represented); i, foramen, through which the fibres of the plug issue; a, a, anterior 

 adductor muscles; a', a', posterior adductor muscles; c,c, protractor muscles; c,' c,' cardinal muscle ; r,T, slidim/ 

 muscle, or anterior visceral muscle of Owen. 



The animal of Discina is extremely delicate and transparent, allowing its internal 

 organisation to be seen through the mantle. The mantle lobes do not adhere to the 

 interior of the shell, which is smooth and polished ; they are bordered with a dense fringe 

 of long horny setcB, which are stiff and extremely brittle, resembling the bristles of certain 

 Annelides (e. g.. Aphrodite).^ The relation of the animal to the perforate and imper- 

 forate valves is the same as in Terehratula, but as in Crania, the only process which can 

 possibly have given any support to the oral arms at their origin, is developed from the 

 centre of the ventral valve. The spiral extremities of the arms are directed towards the 

 lower valve, and not dorsally, as in Crania. Professor Owen has shown in his anatomy 

 oi Discina {Orbicula 'Trans. Zool. Soc.,' 1833), that the valves must have been opened by 

 a pair of oblique, or slidint/ muscles {protractors, c, c, fig. 48, 49) ; besides these there 

 is a slender muscle (attached at the points c, c), similar to the cardinal muscle of 



1 From drawings made by Mr. Woodward : we are indebted to Dr. Gray, for permission to examine 

 the specimens o? Discina, preserved in spirits, in the British Museum. 



2 Professor Owen remarks, in p. 2/8, of his 'Lectures on the Comp. Anat. and Phil, of the Inverte- 

 brate Animals,' 1843: "That tlie margin is fringed with cilia, which are very long in Lingula and 

 Orbicula (Discina). In the latter Brachiopod I found them beset with smaller cilia : these, with the 

 bracliial fringes and microscopic vibratile cilia, which, doubtless, beset the whole surface of the vascular 

 mantle must be the chief agents in introducing the current of sea water within the cavity of the mantle for 

 nutrition and excretion." It is, however, probable, that neither the brachi<il cirri or pallial sefae are 

 themselves vibratile organs. In certain species of Terebratida and Spirifera, the cirri had inflexible 

 calcareoxts supports. (See Pt. Ill, PI. II, fig. 18, and PI. IV, fig. 2.) 



