BIVALVES PI D DOCKS, SHIP-WORMS, ETC. 627 



their neighbours should they lie in their path. They are anchored in their 

 crypts by a byssus. The shell is elongate and when young has two teeth in 

 each valve ; these disappear in the adult. It is one of the most widely 

 distributed of all bivalves, and varies a great deal in shape. 



The GASTROCH^NID^ have wedge-shaped shells that gape widely in front ; 

 the anterior adductor scar is smaller than the posterior ; the pallial sinus is 

 deep ; the gills are as in Saxicava; the siphons naked and united. The 

 ligament is external ; sometimes a single cardinal tooth is present. Some 

 members of the family line their burrows with a shelly tube, to which, 

 however, they are in no way attached. 



Sub-order 6. PHOLADACEA. In this are included the Pholas, or piddock, 

 known to every sea-side visitant, the ship-worm, and their allies. The 

 PholadidsB have white, brittle shell's, roughened like a file on the outside ; 

 they are generally elongate and gape at both ends widely ; but in one genus, 

 Jouanettia, the two valves form an almost spherical shell, with one small 

 aperture. There is no hinge properly speaking, or ligament, the hinge plate 

 being bent back over the umbo and the anterior adductor muscle carried with 

 it so that it lies without the shell. It is by the action of this muscle that 

 the valves are opened, the posterior adductor serving to close them. The 

 little curved, shelly process that arises from beneath the umbo in each valve 

 and projects into the interior serves the purpose of muscular attachment. 

 The animals, which are phosphorescent, have long, united siphons ; the 

 mantle margins are united, except in front, where the large foot, that fills up 

 the opening in the shell, protrudes. The adult animal has additional shelly 

 plates on the back on either side of the umbones. The common Pholas of 

 our shores bores into rocks, the burrows lying parallel to one another ; but 

 some species bore into floating wood across the grain. 



The TEREDINID^E, or ship- worms, have two very small valves widely 

 gaping at both ends, that are borne at the extreme anterior end of the long 

 cylindrical animal. The greater portion of the creatures' length (ordinarily 

 they are a foot long, and sometimes two and a half feet) is taken up by the 

 gills and the fleshy siphons. These last are united to within a short distance 

 of their extremities. At the point where they separate two little shelly 

 pallets, or styles, are found. The true ship-worms bore into timber with the 

 grain, and become so abundant where they once settle that they soon honey- 

 comb and destroy the stoutest balks. Their burrows are lined with shell 

 and sometimes show cross partitioning. One species bores into cocoa-nuts 

 and other woody fruits floating in tropical seas. The tube of the giant 

 Teredo is often a yard long and two in diameter ; it burrows in sand and in 

 the roots of mangroves. 



Sub-order 7. ANATINACEA. In the gills of this group the outer lamella 

 on each side is directed upwards instead of downwards, and its free edge is 

 not reflected. The margins of the mantle are largely united. The shell is 

 usually thin, and pearly within, frequently strengthened at the umbo by a 

 buttress. The hinge rarely has any teeth ; the ligament is generally internal, 

 and more or less divided by a shelly portion or ossicle. The principal 

 families are : PANDORID^E, CHAMOSTREID^, VERTICORDIID^, LYOKSIID^E, 

 ANATINID^E, PHOLADOMYIDJE, and CLAVIGELLIDJE. The Pandoridse are 

 represented in British seas by Pandora, in which the right valve is flat, 



