LINGULA, 203 



" The divaricators are the chief agents in opening them. When they contract, the 

 mnbonal regions of the valves are approximated, and llms pressing forward the fluid in the 

 peri-visceral chamber, their anterior margin is separated. 



" The primary function of the three pairs of adjusters is to keep the valves opposed to 

 each other, or, in other words, to adjust them, and in this respect to compensate for the 

 deficiency of the hinge and condyles. When in full action and in co-operation with 

 the occlusors and divaricator, they likewise assist in closing the valves. The adjustors 

 are the sliding muscles of those authors who believe in the sliding of the valves over each 

 other. The anterior occlusors have had a similar function assigned to them. 



" The peduncular muscle attaches the shell to the peduncle, and has probably the power 

 of moving the former upon the latter.'' 



Some doubt exists concerning the homology of the adjuster muscles (in Lingula), but 

 Mr. Hancock has not expressed any strong opinion on the point ; he thought it likely that 

 the muscles so named in the articulated and unarticulated genera of Brachiopoda were 

 probably homologous, but is ready to admit that he may be possibly mistaken, and, if so, 

 he would not, however, be disposed to change the names, for in both divisions the function 

 of these muscles is to adjust the valves. It is only necessary to keep in view that they 

 are not homologous. 



We will now give Dr. Gratiolet's description of the functions of the muscles, and for 

 which I am indebted to the author himself, who has kindly therein distinguished the effects 

 of simultaneous action and those of the alternate action ; and for the assistance of the 

 reader Mr. Hancock's names have been added within brackets. 



1. Muscles preadducteurs (posterior adductor) and post-adducteurs (divaricators). 



a. Simultaneous action. — The energetically drawing together the valves in their whole 



length. 



b. Alternative action. — When the preadducteurs contract themselves alone, they close 



the shell in front and make it gape behind. When the post-adducteurs 

 contract alone, they close the shell almost completely behind and make it open in 

 front. 



2. Muscles peaussiers verticaux (posterior parietal muscles). 



a. Simultaneous action. — They depress the body behind, cause the internal fluid to flow 

 towards the arms, and consequently come strongly in aid of the action of the 

 " muscles post-adducteurs/' 



3. Muscles obliques transversalement, muscles croises of Cuvier (posterior adjustors) 



a. Simultaneous action. — They energetically draw together the valves. 



b. Alternative action. — Taking for a fixed point the valve which I call the superior 



