MARINE MOLLUSCS 201 



into pieces with a hammer, thus laying open the burrows so that 

 the molluscs fall from their places. 



The Common Piddock (PJiolas dactylus) may be identified by 

 the illustrations, and the other members of the family may be 

 recognised at once by the similarity in structure and habit. The 

 principal species are the Little Piddock (P. parva), the shell of 

 which is wider in proportion to the length, with only one accessory 

 valve ; and the White Piddock (P. Candida), also with a single ac- 

 cessory. In all the above the foot is remarkable for its ice-like 

 transparency. 



There is another genus the Pholadidea the species of which 

 are very similar to pJiolas both in structure and habit. The shells 

 are, however, more globular in form, and are marked by a trans- 

 verse furrow. The gape at the anterior (lower) end is also very 

 wide, and covered over with a hardened plate in the adult. Also, 



FIG. 133. Pholas dactylus, INTERIOR OF VALVE ; AND Pholadidea 

 WITH ANIMAL 



at the posterior (upper) end of the shell is a horny cup through 

 which the siphons protrude, and the latter, which are combined 

 throughout their length, terminate in a disc that is surrounded by a 

 fringe of little radiating appendages. 



In the same family are the molluscs popularly known as ship 

 worms, which are so destructive to the woodwork of piers and 

 jetties, or which burrow into masses of floating timber. Some of 

 these, belonging to the genus Xylophaga a word that signifies 

 ' wood eaters ' have globular shells with a wide gape in front, and 

 burrow into floating wood, nearly always in a direction across the 

 grain. The burrows are about an inch deep, and are lined with a 

 calcareous deposit. The siphons, combined except at the ends, are 

 slender and retractile ; and the foot, which is thick, is capable of 

 considerable extension. 



Other ship worms belong to the genus Teredo, and are very 

 similar in general characters. The shell is small and globular, with 



