48 LAMELLIBRANCHIA. 



Monomyaria probably invariably possess in youtb the two typical adductors 

 of the Dimyarian. 



[The fact that the anterior adductor almost invariably appears 

 before the posterior, not only in the Dimyaria, but also in the 

 Monomyaria, in which latter group it is only a larval structure, 

 might seem to suggest that this muscle was a phylogenetically older 

 structure than the posterior adductor, and that the Lamellibranchia 

 •were originally Monomyarians, not, like the existing Monomyaria, 

 with a single muscle represented by the posterior adductor, but with 

 the anterior adductor alone developed (Jackson). Palaeontology 

 does not, however, bear out this view ; the oldest known Lamelli- 

 branchs found in the Cambrian belong to the Nuculidae and Arcidae, 

 which are typically Dimyarian.] 



The young of Ostrea agrees, not only in the possession of two ad- 

 ductors, but also in other points of its organisation, with the larvae 

 of other Lamellibranchs, but at a later stage it changes from a free 

 to an attached manner of life. 



The larva, which has hitherto swum about freely, possessing two 

 quite symmetrical shell-valves and two adductor muscles, attaches 

 itself by means of a secretion produced by the left lobe of its mantle ; 

 the latter stretches beyond its valve, and, applying itself to the stone 

 or shell to which the valve is to adhere, secretes shelly matter which 

 serves to cement the valve to its support (Huxley, Ryder). In the 

 further development, we now from this time find an inclination to 

 that radial symmetry which can be recognised in the adult Oyster, 

 and which is often found in animals that assume an attached manner 

 of life. The anterior adductor now degenerates and the only remain- 

 ing adductor muscle (the posterior adductor) enlarges and shifts 

 ulmost to the centre of the animal. The anterior part of the body 

 gradually rotates (round its vertical axis) * through an angle of 

 about 90°, so that the mouth, which at first lay very near the free 

 edge of the shell, comes to lie near the umbo. This rotation also 

 explains the great and almost circular extension of the gills and the 

 mantle found in the adult. The condition of the foot in the Lamelli- 

 branchia depends largely upon its use or disuse. In Ostrea, the shell 

 becomes permanently attached at the close of the free-swimming 

 Trochophore stage ; the foot is therefore unnecessary before fixation 



* [Vertical in relation to the substratum, the true transverse axis of the 

 animal. This rotation is possibly due in large measure to tbe degeneration 

 of the anterior adductor muscle and of the velum. — Ed.] 



