724 CLASS xu. 



of mantle, mostly lamellose, two on each side, more rarely pectinate, 

 composed of thin fimbrise or lacinige crowded together. Four lamelli- 

 form tentacles in by far the most, oval or triangular, placed in pairs 

 at the sides of mouth. Two lateral shells incumbent on mantle, 

 conjoined by an elastic ligament at the back. 



On the internal structure of these lamellibranchiate conchifers, 

 what was needful has been already said above. The beard-threads, 

 named byssus, still require a short notice. At the foot of the 

 conchifers (see above, p. 717) there is often situated a bundle of hairs 

 or filaments to which the name of byssus has been given ; sometimes 

 these filaments are united into a common mass ; they are attached 

 in a cavity at the base of the foot by a peculiar substance secreted 

 there (conjunctive matter). From the cavity a furrow runs to 

 nearly the extremity of the foot, and along this furrow is situated 

 the glandular tissue by which the byssus is secreted. REAUMUR 

 maintained that these horny threads were spun just like the web of 

 caterpillars and the nets of spiders. With the assistance of the 

 muscular foot these threads are directed to determinate situations 

 and caused to adhere there firmly by their broad disciforrn extremi- 

 ties ; they grow again if cut off. 



Comp. A. MUELLER de Bysso Acephalorum Acced. Tab. n. Berolini, 

 1836, 4to, and in WIEGMANN'S Archiv, 1837, s. \ 39, Tab. I. 11. Various 

 opinions of writers on these filaments were noticed in the first edition of 

 this Handbook, 11. biz. 37. 



Several conchifers, which are not attached by byssus-fil&ments, 

 grow fast by one of the two valves to rocks or different substances 

 found in the sea. That some of them move freely and get over the 

 ground with their foot has been already noticed (p. 717). The two 

 valves of the shell are separated from each other by the elastic 

 ligament situated at the hinge behind the point (p. 682). White 

 compact muscular fibres, which run transversely from one valve to 

 the other, close the shell on the other hand. These bundles leave 

 impressions on the interior of the shell, two on each valve, the one 

 before, the other behind, if, as in most of these animals, two adductor 

 muscles be present ; if, on the contrary, as in the oyster, there be 

 only one, then only a single muscular impression is to be seen on 

 each valve, situated nearly in the middle. 



POLI made many experiments with regard to the force of these muscles 

 in living conchifers. Thus he states, for instance, that the adductor muscles 

 in the animal of the Noah's shell (Pectunculus) can overcome the resistance 

 of 5olbs., and that more than 7olbs. are required to tear them asunder. 

 The shell weighed only one pound, the animal itself only 4 ounces. 



