THE BIVALVES 



3387 



SUBORDER Pholadacea 



The boring Pholadidce and Teredinidce are the only families contained in this 

 suborder of the group, the former perforating clay, chalk, limestone, and even 

 gneiss. Their shells are always white, thin, but hard and strong, and ornamented 

 with prickly rasp-like sculpture. They gape all round the valves, meeting only at 

 the hinge and the opposite margin. Accessory plates generally occupy the vacant 

 spaces. The valves have no hinge teeth, and are not 

 connected by a well-defined ligament, like most bivalves. 

 The animals have long united siphons, fringed at the 

 apertures, and inclosed in a tough skin, which is often 

 protected by cartilaginous cup-like processes. In the 

 typical Pholas the foot is well developed, and probably 

 forms the principal excavating instrument; the shell be- 

 ing used as a file to enlarge the crypt as the creature 

 grows. Xylophaga and Martesia bore into floating wood. 

 Species of this family are met with everywhere, and 

 about half a dozen occur on the British coasts. In some 

 parts of Europe Pholas is considered a delicacy, and 

 many species are highly phosphorescent. In the sec- 

 ond family the shipworm ( Teredo} is only too well 

 known on account of the amount of damage it does 

 to submerged timber. It matters not whether it be 

 oak, pine, teak, or mahogany which it attacks, soon 

 the timber is riddled through and through, and ren- 

 dered useless. In former times, before the invention of 

 copper sheathing, immense damage was inflicted upon 

 shipping, and the piles of piers and harbors were con- 

 stantly having to be renewed through the ravages of 

 this pest. The Dutch have been great sufferers, and 

 at one time such depredations had been made on the 

 piles which support the dykes of Zealand and Friesland 

 as to threaten them with total destruction. The animal 

 is practically nothing more than an extremely prolonged 

 Pholas. The siphons are of immense length, in some 

 cases from two to three feet long, united except toward 

 the ends. On the contrary, the body itself containing 

 the principal viscera is small, and protected by a 

 globular, bivalved shell, open both in front and behind. 

 The gills are narrow, elongate, and prolonged into the 

 branchial siphon. The siphons secrete a shelly lining 

 to the burrow, and at the point where they separate 

 there are a pair of calcareous plates, or pallets as they 

 are termed, probably used as a means of defense, in 

 closing the tube after the siphons have been retracted. 



SHIPWORM (Teredo) AND 



LARVA. 



ITS 



