144 



BYSSIFEROUS MOLLUSCA. 



nexed figure may assist you in comprehending the process. 

 24. (^^S- -4.) There d repre- 



sents the organ by which 

 the byssus is fixed ; c, the 

 byssus itself, with some of 

 its fibres glued to the stone ; 

 and a, b, the muscles by 

 whose contraction the foot 

 is pulled within the shell 

 from its state of extension. 

 The structure of the foot, 

 or spinner,* as it might 

 more properly be called, is 

 exquisitely adapted to its 

 purpose. Situated be- 



tween the mouth and the 

 byssus, it is distinguished 

 by its tongue -like shape, 

 its flexibility, its powers of elongation and contraction, and, 

 in this species, by its deep violet colour. There is an open 

 furrow traced along its middle, capable of being converted 

 into a closed canal at will, down which the gummy fluid, from 

 which the threads are spun, flows. This comes from a gland, 

 or glandular parts, situated at the base of the foot, where it 

 is secreted, and wells out when required, being formed into 

 threads in the furrow or canal. f You perceive from the figure 

 that the byssus of the common mussel resembles a fibrous 

 root, originating in a single stalk, that di\ddes in a very irre- 

 gular manner ; but the byssus of the Modiola, and more espe- 

 cially of the Pinna, is more accurately represented by a camel- 

 hair pencil. In the former the root of it is invested with a 

 simple fleshy sheath ; in the Pinna the sheath contains five 

 muscular leaflets, which act as partition-walls to four la- 

 minae of nearly the same size and flgure as the others 

 (Fig. 25), but composed of a close tissue of interwoven 

 threads, by the separation and unravelling of which, and 



just as a boy's leather sucker is made to fix itself to a flat stone {Select 

 Works, i. 78) ; but this is not the case : the threads of the mussel are at- 

 tached by a close glue or cementation. 



* From the foot of other bivalves it is distinguished by its position and re- 

 lation to the other viscera, and its mode of connection with them. Poli and 

 Van Beneden call it the "languette," or tonguelet ; Lister, the "lingua," who, 

 with some others among the earlier naturalists, mistook it for a sexual organ. 



f " This gland, of which the existence is erroneously denied by Blainville, 

 is of a brown granular appearance : it may readily be found in tlie Mytilus, 

 or Modiola, lying upon the nervous ganglion of the foot. Its duct opens into 

 the bottom of the groove situated on the posterior surface of that organ." — 

 Garner in Mais. Nat. Hist. n. s. iii. 126. 



