SEA MUSSEL MYTILUS EDULIS. 133 



The two valves are attached at their anterior dorsal edges by means of a hinge 

 plate over which the periostracum extends from one valve to the other. Teeth are 

 present at the anterior end of the hinge in numbers varying from one to six, the most 

 common number being three or four. The teeth of one valve are so arranged that they 

 fit into the depressions between the teeth on the opposite side. In size they are small, 

 rarely over a millimeter in length, and in form they are conical pointed knobs or wedge- 

 shaped lamellae (fig. 103, T). 



The ligament (fig. 102, Lig), a straight, brownish colored, elastic rod, lies between 

 the two valves just beneath the hinge band and helps to unite the shell edges. In cross 

 section it presents the form of an ellipse with its long diameter lying in a horizontal 

 position. It is bounded laterally by parallel ridges, the ligament ridges (fig. 102, LR), 

 which have a very characteristic structure. They are chalky white in color and per- 

 forated with numerous pores. Each ridge terminates in a fine point both anteriorly and 

 posteriorly. The median surfaces are 



concave to fit snugly against the liga- ^^^2^^>v^-- //£ 



ment when the shell is normally open; ^tffZ S^sJ 



consequently when the valves are closed 

 by contraction of the adductor muscles 

 the ligament is compressed and its elas- 

 ticity tends to counteract the action of 



the muscles (fig. 107). Asa. esult, when §p j >^---PL 

 the adductor muscles relax the ligament "lik / J • 

 forces the shell open again. This ex- 

 plains why the shells of dead mussels are 

 always open. \^ ^^. -Per 



The umbo is at the anterior end of 

 the shell and forms a sharp beak, off the 

 ventral side of which may be found, \QJ 



often hidden by the periostracum, a _ _ ^. , . „. .. .... 



J L Fig. 107. — Cross section of a shell 111 hinge region in diagrammatic 



Special Structure called the lunula (fig. lonn to show relation of ligament to hinge plate and valves. HL, 



IO6, LU, Opp. p. 132), which bears a hinge ligament; HP. hinge plate; ZJ?, ligament ric^c. XL, na- 



. , . ... creous layer; Per, periostracum; PL, prismatic layer. 



definite relation to the hinge teeth. 



It consists of a series of semicircular furrows and ridges which run out peripherally from 

 the teeth and terminate at the umbo. Each furrow corresponds to a tooth; and each 

 ridge, to a depression between the teeth. The lunula is conspicuous only in individuals 

 where the teeth are comparatively large in size and number. 



When the valves of a normal shell are shut they form a complete closure. 

 If, however, they are first treated with a solution of potassium hydroxide, which removes 

 all the periostracum, it will be found when they are closed that there are two places 

 where the edges fail to come in contact. On the ventral side in the middle of the shell 

 there is a fissure through which the byssus may project, the byssus cleft (fig. 106, ByC). 

 In the normal shell this cleft is hidden by a fold of the periostracum which incloses the 

 marginal blood sinus. The structure is such as to act as a cushion which presses against 

 the byssus when the shell is closed. Corresponding to the byssus cleft on the dorsal 

 side in the hinge region is another opening between the valves, the ligament cleft (fig. 

 105, LC, opp. p. 132). In the complete shell this depression is covered externally by the 

 periostracum and internally by the underlying ligament. 



