328 



GENERAL SUMMARY TO 



no muscles for such an object are required, consequently none arise from the 

 lateral portions of the valves, as in Lingula. But in an extinct group, the Trimerellida, 

 which seem to be somewhat intermediate in character between the Tretenterata and 



Clistenterata, have been found certain scars which 

 appear to have been produced by rudimentary lateral 

 muscles ; still it is doubtful (considering the shells are 

 furnished with teeth, though rudely developed) whether 

 such muscles enabled the valves to move forward and 

 backward upon each other, as in Lingula} We do not 

 yet possess any reliable observations as to the manner 

 in which Discina opens its valves ; but Barrett, who 

 observed Crania in life, informs us that the valves open 

 by moving upon the straight hinge without sliding the 

 valve. There are muscles connected with other portions 

 of the animal in both groups, such as the parietal mus- 

 cles, strongly defined in the Tretenterates, and distinctive 

 peculiarities of the peduncle, but the limited space at 

 our disposal will not admit of our entering upon further 

 anatomical details. 2 



We are also indebted to Professor Morse and to 

 Professor W. K. Brooks % for some valuable observations 

 on the muscular arrangement in the young ages of 

 Lingula (Glottidia) pyramidata, Stimpson, which animal 

 the former met with in great abundance at Fort Wool in 

 the north of Hampton Roads, Baltimore. Brooks says in 

 his memoir, p. 56 : " In the youngest stage observed, 

 only a single pair of muscles are present. They are 

 situated on each side of the oesophagus, close to its union 

 with the stomach, and traverse the body-cavity from one valve to the other. They 

 are the muscles ' h ' of Davidson's article " Brachiopoda " (in the ' Encyclopaedia 

 Britannica,' ninth edition ; and of King's figures, A and B, here appended) ; by their 

 contraction they bring the two valves together, compressing the body between them. 

 The separation of the valves appears to be accomplished at this stage entirely by the 

 muscular integument. When the muscles ' h ' are relaxed, and the lateral walls of 

 the body contracted, the width of the body is diminished and its thickness increased, 

 and the valves thus pushed apart At a somewhat later stage the 



Lingula anatina. Diagram showing 

 the muscular system, after Hancock. 

 The letters indicate the muscles, as in 

 figures A and B, p. 327- 



A, Ventral valve; b, dorsal valve; p, 

 peduncle; e, heart (?); a, alimentary 

 tube ; z, anal aperture. 



1 See a paper by Davidson and King on the " Trhnerellidce," ' Quart. Journal Geol. Soc.,' vol. xxx, 1874. 



2 The details above given on the muscles were published by myself in the article " Brachiopoda," 

 ' British Encyclopaedia,' 9th edition, p. 192, 1876. 



"The Development of the Brachiopoda," ' Chesapeake Zoological Laboratory,' 1878. 



