DR. T. DAVIDSON ON EECENT BEACHIOPODA. 



45 



of an outer and an inner layer, between wliicli are situated tlie blood-channels or lacunae. 

 There are four principal arterial trunks in the dorsal lobe of the mantle, and these run 

 direct to the front and bifurcate at intervals (see PI. VII. figs. 15, 16 d). The aperture 

 of the mouth is situated in the middle line between the pallial lobes, and on each side 

 of it is a prolongation of the body provided with ciliated teatacula. 



The muscles have been minutely described by the three anatomists above named. 

 Five or six pairs are connected with the opening and closing of the valves, or with their 

 attachment to, or movements upon, the peduncle. First of all the adductoi- or occlusor 

 consists of two muscles, which, bifurcating near the centre of the cavity, produce a large 

 quadrangular impression on the internal surface of the dorsal valve. The function of 



FiE 



Waldhwnia JldvcKCtns (after Hancock). 



M, ventral Talvc : X, dorsal valve ; J, loop : r, mouth ; :. extremit)- of intestine : a, adductor : f, divaricators ; 

 (■', accessory divaricators; h, ventral adjustors; h\ peduncular muscle; h" , dorsal adjustors ; P, peduncle. 



this pair of muscles is the closing of the valves. Two other pairs have been termed 

 dkarlcatoi's by Hancock, and have the function of opening the valves. The divaricators 

 proper arc stated by the same eminent authority to rise from the ventral valve, one on 

 each side, a little in advance of and close to the adductors, and after rapidly diminishing 

 in size, become attached to the cardinal process, a space or prominence between the sockets 

 in the dorsal valve. The accessory divaricators are a pair of small muscles which have 

 their ends attached to the ventral valve, one on each side of the median line, a little 

 l)ohind the united basis of the adductors, and again to the extreme point of the cardinal 

 process. The two pairs of muscles, apparently connected with the peduncle and their 

 limited movements, have been minutely described by Hancock as having one of then- 

 extremities attached to this organ. Tlie dorsal adjustors are attached to the ventral 

 surface of the peduncle, and are again inserted into the hinge-plate of the smaller valve. 

 The ventral adjustors are considered to pass from the inner extremity of the peduncle, 

 and to become attached by one pair of their extremities to the ventral valve, one on 

 each side of and a little behind tlie expanded base of the divaricators. The function ot 

 these muscles is not only that of erecting the shell, but also that of attaching the 



