of the Fishery Board for Scotland. 221 



IV.— THE SPAWNING, GROWTH, AND MOVEMENT OF THE 

 MUSSEL {MYTILUS JWULIS, L.), HORSE-MUSSEL 

 (MODIOLUS MODIOLUS, L.) AND THE SPOUTFISH 

 {SOLEN SILIQUA, L.) By H. Chas. Williamson, M.A., D.Sc, 

 Marine Laboratory, Aberdeen. 



(Plates XVI.-XX.) 



CONTENTS. 



PAGE. 



The Development of the Reproductive Organs, - - - 221 



Spawning, -------- 228 



Growth, -------- 233 



Movement, -------- 242 



Structure of Foot, ------- 244 



Structure of Byssus-Pit, ------ 247 



The Byssus-Stem, ------- 250 



Formation of Byssus-Stem, ----- 251 



Growth of the Spoutfish, ------ 251 



Literature, -------- 252 



Explanation of Plates, .--.-- 253 



During the past three years I have carried oh certain experiments 

 bearing on the natural history of the Mussel, and the results are 

 embodied in the following pages. 



There is an extensive literature on the mussel and its culture, and, 

 notwithstanding that fact, there are various points in its life-history 

 which require further elucidation. While the development has been 

 studied by several authors, especially by Wilson, there are still blanks 

 in the early life-history. And many of the causes which influence the 

 well-being of the mussel in difl'erent localities are not clearly defined. 



The facts which I am able to communicate extend our knowledge of 

 the spawning, growth, and movement of the shell-fish. 



I shall first of all discuss the development of the reproductive organs, 

 and the appearance of the mussel in difl'erent months of the year. 



llie Develojwient of the Reproductive Organs. 



In the quiescent period, according to Wilson, the mantle lobes are 

 delicate and transparent. "The foUiclep are racemose outgrowths of 

 th^ minor genital canals, . . . and they may be observed pushing 

 their way in the mantle until they reach nearly to the pallial muscle. 

 With a little experience it is possible to detect the sex of the specimen, 

 the darker orange tinge and the less distinctly botryoidal arrangement of 

 the female, as contrasted with the male glands, being usually marked. 

 The colour may be almost absent in comparatively young forms of both 

 sexes. The genital ducts lie mostly toward the outer side of the mantle. 

 Where the branches enter the follicle they may have ciliated columnar 

 epithelium on the one side and germinal epithelium on the other. 

 The genital tube is one of two common ducts which pass to the outside." 

 p 



