CONCHIFERA. 47 



are only slightly separated, are overlapped by two 

 lobes, which are everted and provided with very 

 active vibratile cilia : the young swim by means 

 of these lobes.* The embryos of Teredo have been 

 also observed swimming freely about by means 

 of a foot-like organ, which protrudes between 

 the valves, and presents an active ciliary move- 

 ment.f 



In order to apprehend aright the technical de- 

 scriptions of families and genera in this Class, it 

 may be needful to explain the principal terms used. 

 The valves are hollow cones, the points of which 

 are bent over towards one side. In some cases the 

 conical form can scarcely be recognised, except by 

 comparison with others, such as Isocardia, in 

 which the points are much developed and curled 

 in a spiral manner. The point is termed the beak 

 (umbo) of the valve. 



The side towards which the beaks twist is the 

 front side, and on this side, beneath the beaks, is 

 often a depressed space termed the lunule, while on 

 the hinder side is placed the ligament. The lunule, 

 umbo, and ligament, constitute the dorsal or back 

 border; the opposite side to this, or that which 

 gapes, is of course the ventral border. The length 

 of a shell measures a line drawn from the dorsal to 

 the ventral border ; the width, therefore, is a line 

 at right angles to this ; and the thickness is mea- 

 sured from the centre of one valve to that of the 

 other, through the body of the animal. 



The siphons, when present, project from the 

 hinder side ; the more ventral one is the receiving 

 or branchial siphon, the more dorsal the discharg- 

 ing or anal. The foot usually points towards the 



* Loven, Arch. Skand. i. 155. + Siebold, Op. cit. 200. 



