126 ANATOMY OF THE LAMELLIBRANCHIATA. 



is no cartilage ; ' but a process of the mantle overlaps the 

 beaks of the valves, and secretes a calcareous plate upon them. 

 This reflected portion of the mantle covers the anterior mus- 

 cle, which here goes from beak to beak ; in the calcareous 

 plates in the P. dactylus, without the insertion of the muscle, 

 is an external row of large and an internal one of small cavi- 

 ties, into which are inserted corresponding fimbriations of the 

 reflected portion of the mantle. This fleshy process is pro- 

 tected and covered by several thin calcareous plates, imbed- 

 ded between it and the cuticle : there are four of these in the 

 P. dactylus, but one in the P. candidus, P. conoides, &c. 

 There are likewise two spoon-like processes in the interior of 

 the valves, below the beaks ; secreted in two reflections of 

 the mantle, and giving attachment to a few of the fibres of 

 the foot. The Teredo has the valves joined by muscular 

 fibres alone, as has the Myastropha. The teeth are of infi- 

 nite diversity, in shape and position, and merit a more minute 

 examination than they have hitherto had. 2 



MUSCULAR SYSTEM. 



Many of these animals are immoveably fixed to the spots 

 on which they are found. The oyster, for instance, in the 

 young state, secretes the calcareous matter of the left valve 

 on rocks, &c, and only ceases to do so when a firm attach- 

 ment is formed. Other species are attached by a set of horny 

 filaments called the byssus. This is formed from the secretion 

 of a bilobed gland, situated within the base of the foot. This 

 gland, of which the existence is erroneously denied by Blain- 

 ville, is of a brown granular appearance ; it may readily be 

 found in the 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. Its 

 fluid secretion is moulded in this groove, and the thread, 

 which rapidly hardens, is fastened at one end to the tendi- 

 nous base of the foot, and at the other, by an expanded ex- 

 tremity, to the rocks to which the animal adheres. On rocky 

 shores we see how firmly and immoveably the common 

 muscles are bound by these threads. The Modiola discors 

 fixes itself to the cartilaginous tunics of Phallusia and other 

 Tunicata, and becomes buried in them, the anal extremity 

 only projecting. Some species of Pecten are fixed by the 

 spinous processes of their valves, some by a byssus, while 



1 In the P. candidus, however, the author finds one, between the two 

 small spinous processes. 



8 See a paper on the hinge of Bivalves by Wood, ' Linn. Trans.' vol. 6. 



