6i8 MOLLUSCA CLASS ILPELECYPODA. 



some forms, like the oyster, only one such, the hinder one, remains. 

 Bivalves with two adductor scars in each valve are termed Dimyaria, the 

 others Monomyaria. The reduction and disappearance of the anterior 

 adductor may be accounted for by the more rapid growth of the latter half 

 of the body, which thus tends to bring the two muscles and the hinge into 

 line, when the anterior adductor, being less and less required, is propor- 

 tionately reduced and ultimately disappears. In Pinna the hinge is reduced 

 to the lowest limit instead. Running from one adductor scar to the other 

 in the Dirnyaria, and at a short distance from the ventral margin of the shell, 

 is a shallow groove, the pallial line, which marks the attachment of the 

 muscular mantle. In a large group of the bivalves this line, instead of 

 running in a continuous curve, makes, shortly before it reaches the hinder 

 scar, a sharp bend back towards the centre of the valve, forming a bay or 

 sinus called the pallial simis. Its presence indicates that the animal had 

 retractile breathing tubes or siphons, and to these further allusion will be 

 made later on. Other small scars of lesser importance are generally present. 

 The terms employed in designating the different parts will best be gathered 

 from the accompanying diagrams (Fig. 5). 



No bivalve mollusc possesses a head, and hence the term Acephala, or its 

 equivalent Lipocephala, has sometimes been applied to the class. The animal 

 is enclosed between the two lobes of the thin mantle, one lying on either 

 side of it immediately under the shell. These lobes are attached along the 

 back and reach out to the margin of the valves. In the more primitive 

 forms the mantle margins are quite simple, and open from the front round 

 the ventral edge to the back ; but many different modifications take place in 

 the higher forms. Instead of the margins being simple, there may be folds, 



Fig. 6. DIAGRAM TO ILLUSTRATE THR DIFFERENT FORMS OF MANTLB OPENING AND TTTB 

 FORMATION OF TUB SIPHON. 



A, Mantle open all round. 



B, Mantle margins appl.ed to each other except at e (exhalent aperture) and t (iahalent aperture) 



and where /(the foot) protrudes. 



C, Mantle margins grown together at 1. 



D, Mantle margins grown together at 1 and 2. 



E, Mantle margins prolonged into siphons, e and t (as before), united at 1 and 2. 

 F v Siphons united, alao mantle margins at 2 and 3. 



thickenings, protuberances, tentacles, various glands, and even eyes. Nor are 

 the margins always free ; indeed, in extreme cases, they are united nearly the 

 whole way round. There are various stages (Fig. 6) : counting the open one 

 as the first, in the second the currents of water, instead of being drawn in all 

 the way round, the edges of the mantle are kept close together except where 

 the foot protrudes and at two points at the hinder end, one of which (i) serves 

 to admit the fresh water to the gills and food to the mouth, and is called the 



