THE BRITISH FOSSIL BRACHIOPODA. 



integument at the posterior end of the body is much thickened, and soon a band of 

 muscular fibres, ' r,' makes its appearance in it upon the median line, and runs directly 

 from one valve to the other. At subsequent stages the fibres increase in number, 

 and form a well-marked muscular bundle, which is the muscle ' g.' When contracted at 

 the same time with the muscles ' /i,' this muscle serves to flatten the body still more, 

 and this brings the two valves closer together ; but when the muscles ' h ' are relaxed 

 and the muscle ' g ' alone contracted, the posterior margins of the valves are brought 

 together and their anterior edges thrown apart, the body furnishing the fulcrum around 

 which they turn." Some time after, " Three pairs of muscles make their appearance 

 in the body-cavity at the sides of the stomach, two on the dorsal and one on the ventral 

 side. ... In a dorsal view of the same larva the dorsal ends of the two pairs of 

 muscles are seen. They are attached to the shell a little outside the limits of the body, 

 and the integument bends out with them. One pair of them runs forward and downwards, 

 and apparently unites with the ventral ends of the muscle ' h,' for no corresponding 

 independent muscles can be seen in a ventral view. In the ventral end, 'p,' the 

 muscle * h ' is plainly seen to be divided into two by a longitudinal line, and a com- 

 parison with the ' Encyclopaedia ' figures will show that the outer half is the ventral 

 end of the muscle ' u ' of Brooks's figure. This muscle apparently corresponds to the 

 two muscles ' k ' and ' / ' of the adult, and it probably divides into two at a later stage. 

 It would appear, then, from these observations that the muscles 'j t * ' k' and '/' of 

 the adult Lingula are derived from the muscle ' ft,' some of its fibres separating from it 

 at the ventral end to form the muscles 'j,' but remaining for some time united to it 

 dorsally ; while another bundle separates from the dorsal end, but remains attached for 

 some time ventrally, and gives rise to the muscles ' k ' and ' /.' 



" According to King and Davidson these three pairs of 

 muscles are adapted to move the valves forward and back- 

 wards upon each other ; but as my observations show that 

 neither the larva nor the adult does slide its valves backwards 

 and forwards to any marked degree, it seems more probable 

 that they, together with the pair of muscles next to be de- 

 scribed, cause the rotation of one valve upon the other around 

 the body as a centre. In the dorsal view given by Brooks a 

 pair of muscles, ' u,' are shown attached to the shell close 

 to the muscles last described, and running backwards and 

 inwards under the stomach. These are the muscles ' i* which 

 in the adult cross each other under the digestive organs, and 

 are the principal causes of the rotation of one valve upon 

 another, which has been described by Semper 1 and Morse." 



Professor Morse says that he observed that Lingula had the power of moving over the 

 1 'Zeitschrift fiir wissens. Zool.,' vol. 2, 18. f >9. 



Lingula pyramidata, from life 

 (after Morse). 



