212 



DE. T. DAVIDSON ON RECENT BEACHIOPODA. 



23. 



It was long a question under discussion whether the animal could really displace its 



valves sideways when about to open the shell ; l)ut this has been actually observed by 



Profs. Semper, Morse, and Brooks, who saw the animal 



perform the operation. They mention tliat it is never 



done by jerks, as the valves are first always pushed to 



one side several times and back again on each other, at 



the same time opening gradually and in the transverse 



direction till they rest opposite to one another and widely 



apart. Those who have not seen the animal in life, they 



observe, and did not believe in the possibility of the valves 



crossing each other with a slight ol)liquity, would not 



consent to the appropxiation of any of the muscles to that 



purpose, and consequently attributed to all the lateral 



muscles the simple function of keeping the valves in 



opposite position or holding them adjusted. Prof. King 



states, in the paper already quoted, that "the umboual 



muscle," {g), "is in no way fettered by the pedicle, or any 



other part. The shell is not only edentulous, but its 



hinge-margins are widely and totally separated from each 



other. The beaks have their margins persistently apart, 



even when the umbonal muscle is mostrigid. The post-latero- 



parietals are highly muscular, necessarily permitting an 



unusual play of motion between the valves at their pos- 

 terior extremity. The ordinary muscles {h,j, h, I), princi- 

 pally, are limited to the middle third of the valves All 



these structural peculiarities are reciprocally related, and 



they are strictly consistent with the office herein ascribed 



to the transmedian muscles." 



" The mantle-lobes forming the dorsal (upper) and ven- 

 tral (under) surfaces of the chamber are well characterized 

 by the vascular system. Both lobes are traversed by a pair 

 of primary vessels that run foi'ward from the anterior 



parietal, a member from each of its sides : gradually approximating in their progress, 

 without becoming united, they terminate, the dorsal pair at about a quarter of an inch, 

 and the ventral pair at about twice this distance, from the margins of the chambers. 

 Prom their inner side numerous secondary vessels strike inwardly, Avith a backward 



curve, and meet in the middle line of the valves Although the setal band, as it 



may be termed, runs along the pallial margins in their entire extent, the course of the 

 seta3 in the ventral valve is interrupted in one j)art — that is, in the region of the pedicle : 

 there is no interruption in the opposite part of the dorsal valve." 



The brachial organs of Lincjula anatina are described by Mr. Hancock (at p. 810 

 of the memoir already quoted) to be " strong and fleshy, and rise from the back of the 

 pallial chamber in the usual manner. They are entirely without calcareous support, even at 



Lingula anatina. 



Diagram, after Hancock, showing the 

 muscular system. The letters 

 indicate the muscles as in fig. 22, 

 A and B. 



A, ventral valve ; B, dorsal valve ; j), 

 peduncle ; a, alimentary canal ; 

 z, anus. 



