BYSSIFEROUS MOLLUSCA. 143 



be kept in repair by mortar. The corporation, therefore, 

 keep boats in employ to bring mussels to it, and the inter- 

 stices of the bridge are filled by 

 hand with these mussels. It Fig. 23. 



is supported from being driven 

 away by the tide entirely by the 

 strong threads these mussels fix 

 to the stonework; and by an 

 act, or grant, it is a crime Hable 

 to transportation for any person 

 to remove these mussels, unless 

 in the presence and by the con- 

 sent of the corporative trus- 

 tees."* 



The byssus consists of a bun- 

 dle of horny fibres or threads, 



connected to the animal within the shell on the one hand, 

 and to the rock on the other. How this connection is 

 effected was first discovered and explained, in his usual 

 copious and clear manner, by Reaumur. By placing mus- 

 sels (Mytilus edulis) in vases of sea-water, he found the 

 following to be their manner of proceeding. Opening 

 their valves, the foot was first protruded, and, with various 

 strains and stretches, gradually thrust out, until at length 

 the elongation was carried to the desired extent, some- 

 times to fully two inches. It was now employed in feel- 

 ing or testing all the objects within reach, directed or to 

 the right or left, backwards or forwards. After all this 

 prelude, to ascertain, apparently, the security of the in- 

 tended holdings, the point of the foot is settled and re- 

 tained for a short time on the chosen spot, when it is again 

 suddenly removed, and immediately withdrawn entirely 

 within the shell, leaving behind a thread that reaches 

 from the spot to the base of the foot. By many repe- 

 titions of this operation, carried on patiently day after day 

 (for not above four or five threads are spun in the twenty- 

 four hours), and by attaching the disk-like extremities of 

 the threads to different places, the mussel at last com- 

 pletes its cable and secures a safe anchorage.-)- The an- 



* Drummond's Letters to a Young Naturalist, p. 39. Patterson's Intr. 

 Zoology for Schools, i. 170. 



f The threads are frequently affixed in straight lines, and at short but 

 equal distances ; and the circular disks appear then very plainly, especially 

 on the surface of a bivalve shell. Leeuwenhoek asserts that the disks adhere 

 to the foreign body partly by the pressure of the atmosphere, and partly be- 

 cause no air or water can gain admittance bet ween the stone and the disk, 



