ORDER SESSILIA ; BALANUS AND ITS ALLIES. 295 



ture ; and the valves do not cover the whole surface. In the 

 Otion, there are only two small valves, with the rudiments of 

 three others. 



ORDER II. SESSILIA. 



830. The shell of the Balanus^ commonly known as the "Acorn- 

 shell," and met with almost everywhere on our shores, is formed 

 on a very different plan. It consists of six principal valves, of 

 a triangular shape, the base of each triangle 

 being at the bottom, and the apex or point at 

 the top. Within these, however, is another 

 set of six smaller triangular valves, arranged in 

 ??$^ j 118 ^ the opposite direction, so as to fill up the 

 space between the preceding, and to give to the 

 whole shell somewhat the form of a cone with 

 the top cut off. The aperture of the shell is closed by an 

 operculum, or lid, composed of two or four pieces ; this the animal 

 can apply to the orifice, in such a manner as to close it com- 

 pletely, when its body is entirely drawn within the shell. 

 The growth of this shelly box is provided for by additions to the 

 edges of the several plates of which it is composed ; these addi- 

 tions increasing the size of the whole, without altering its form. 

 Between the internal and external layers of each valve, there is 

 a regular cancellated, or minutely-chambered structure, like that 

 of Bone (ANIM. PHYSIOL. 47) : and this appears to be filled, 

 as in it, with a sort of marrow. No structure resembling this 

 has been found in any Mollusca. It is especially developed in 

 the Coronula, a genus allied to the Balanus, which is only found 

 attached to the back of Whales and Turtles. 



