THE YOUNG NATUEALIST. 



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interlocked together ; the Ligament, which is 

 n elastic horny substance, joining the two 

 alves together along some portion of the 

 dorsal margin ; and the Cartilage, an elastic 1 

 substance, which is compressed by the closing 

 f the valves, and thus forces them open 

 whoa the muscles used to close them are 

 relaxed. The muscles by which the valves 

 are closed, and by which also ths animal is 

 ttached to the shell are called the Adductor 

 muscles. Some Bivalves are furnished with 

 one central adductor muscle, others have 

 two placed near to the opposite ends of the 

 alves. The place of attachment of these 

 muscles may be seen on the inner surface of 

 the valves, and are called the Muscular Im- 

 pressions : where there are two adductor 

 muscles there may generally be seen also a 

 ine, either curved or sinuous, connecting 

 connecting them : this is called the Pcdlial 

 mpression, being the impression of the mantle, 

 There is frequently a portion of the shell, at 

 the anterior side of the Umboes, rather sunk 

 below the other part, and tilling up the space 

 between the curved line of the valves and the 

 dorsal margin : this is called the Lunule. A 

 similar part when occurring posterior to the 

 Umboes is called the Dorsal Area. The 

 animals of the Bivalves or Acephala have 

 already been described as having no head, 

 and as being all water dwellers. Their body 

 s enclosed between the two lobes of the 

 mantle, which forms a sort of pouch, and is 

 covered externally by the shell. The mantle 

 s sometimes open, sometimes closed, execept 

 where the foot and the Siphonal Tubes emerge. 

 The Siphons or Siphonal Tubes are the openings 

 or reception aud rejection by the animal ; 

 hey are at the posterior side, and are some- 

 imes long and capable of being protuded or 

 vithdrawn within the shell ; sometimes they 

 ire short, and sometimes merely openings in 

 he mantle ; in some they are divided, form- 

 ng two separate tubes ; in others they are 

 mited, or partially so. The Foot is the 

 "principal organ of locomotion, and by means j 

 it it the mullusc is enabled not only to move 



about from place to place, but also to bore its 

 way through the sand, mud, or other material 

 in which it makes its dwelling. Some Bivalves 

 attach themselves to some external object by 

 what is called a Byssus, which consists of a 

 number of strong threadlike fibres. In the 

 Bivalves we have only one tribe called 

 Lamellibranchiata (Lamella gilled); all having 

 breathing organs adapted for the respiration 

 I of water. They are divided into two orders, 

 Unimusculosa and Bimusculosa ; the former 

 \ comprising those which have the valves 

 i attached by one central adductor muscle ; 



the latter those which are provided with two 

 J muscles placed at or near the ends of the 

 . valves. The Unimusculosa consists of only 

 one family, Mytilacea ; while the Bimusculosa 

 contains two, namely, N aides and Cardiacca. 



Mytilacea.— This is the family of the 

 true mussells, of which we have only one 

 i fresh water representative. The shell is 

 ; triangular, the umbo being at one end, and 

 | forming an acute angle ; the valves are 

 ; attached by a compound muscle near the 

 posterior margin. Proceeding from the foot 

 is a Byssus of strong thread-like fibres, by 

 which it attaches itself to some external object. 

 The genus is named Drcissena. 



Naiades. — The shell is oval and of large 

 size, the animal attached by a pair of adductor 

 i muscles, one at each end of the valves ; the 

 I mantle is open except at the margins of the 

 | siphonal openings, which do not protrude, the 

 foot is large and by its aid these fresh-water 

 1 mussels can make their way quickly through 

 | the mud in which they live. The family 

 combrises two genera, Anodonta and Unio. 



Cardiacea. — The shell is small and 

 round ; the animal attached by a pair of 

 | muscles as in the preceding family, the mantle 

 i is not so fully open as in Naiades ; the siphons 

 are rather wholly or partially united. There 

 are two genera Pisidum and Cyclas. 



These are all the families of Fresh-water 

 Bivalves, the Genera and Species will next 

 be described. 



(To be continued.) 



