﻿200 T. Davidson — What is a Brachiopod? 



duce a large quadruple impression on the internal surface of the 

 small valve, and a single divided one towards the centre of the large 

 or ventral valve. The function of this pair of muscles is the closing 

 of the valves. Gratiolet, who has likewise described with great 

 minuteness the muscles of the Brachiopoda, informs us that those 

 which close and open the valves were the only ones known to Pallas, 

 but that he clearly defined their positions and functions. The same 

 was done by Blainville and Quenstedt, but the absence of good 

 figures caused much uncertainty to prevail. This deficiency was 

 subsequently supplied by Hancock's and Gratiolet's admirable illus- 

 trations.^ Two other pairs have been termed divaricators by Hancock 

 or cardinal muscles (= muscles diducteurs of Gratiolet), and have 

 for function the opening of the valves. The divaricators proper are 

 stated by Hancock to arise 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, according to the same authority, a pair of small 

 muscles which have their ends attached to the ventral valve, one on 

 each side of the median line, a little behind the united basis of the 

 adductors and again to the extreme point of the cardinal process. 

 Two pairs of muscles, apparently connected with the peduncle and 

 its limited movements, have been minutely described by Hancock as 

 having one of their extremities attached to this organ. The dorsal 

 adjusters are attached to the ventral surface of the peduncle, and are 

 again inserted into the hinge-plate in the smaller valve. The ventral 

 adjusters 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, the 

 expanded base of the divaricators. The function of these muscles, 

 according to the same authority, is not only that of erecting the 

 shell, but also that of attaching the peduncle to the shell, and thus 

 to effect the steadying of it upon the peduncle. 



Gratiolet describes the peduncle with great care, and states that it 

 is composed of two portions : — 1. A horny sheath formed of con- 

 centric epidermic layers, very analogous to that which Prof. Vogt 

 described in Lingula. 2. A fibrous stem enveloped by the sheath. 

 This stem, formed of tendinous fibres, is fixed by its free extremity 

 to different marine objects ; the other end passes through the foramen 

 into the shell, and ends in a bulbous-shaped extremity. 



Such is the general arrangement of the shell muscles in the division 

 composing the articulate Brachiopoda, making allowance for certain 

 unimportant modifications observable in the shells composing the 

 diffei'ent families and genera thereof. 



In the Tretenterata, of which Lingula may be cited as an example, 

 the myology is much more complicated, and anatomists have differed 

 considerably in their respective views concerning the function of 

 some of the muscles. They have been described by Vogt, Hancock, 

 Gratiolet, and others, and more recently by King, whose views I 

 ' Journal de Couchyliologie, Octobre, 1857. 



