The Common Piddock (_Pholas 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 accessory. In all the above the foot is remarkable for its ice-like transparency. [Illustration: FIG. 133.--_Pholas dactylus_, INTERIOR OF VALVE; AND _Pholadidea_ WITH ANIMAL] There is another genus--the _Pholadidea_--the species of which are very similar to _pholas_ both in structure and habit. The shells are, however, more globular in form, and are marked by a transverse furrow. The gape at the anterior (lower) end is also very wide, and covered over with a hardened plate in the adult. Also, 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. [Illustration: FIG. 134.--THE SHIP WORM] [Illustration: FIG. 135.--1. _Teredo navalis._ 2. _Teredo norvegica_] Other ship worms belong to the genus _Teredo_, and are very similar in general characters. The shell is small and globular, with a wide gape at both ends, and consists of two three-lobed valves with concentric furrows. It is so small in proportion to the size of the animal that it encloses but a small portion of the body, and lies at the bottom of the burrow, which is of considerable length--often from one to two feet. The animal is very wormlike in form; and although the shell is so small, yet all the internal organs are enclosed by it. The mantle lobes are united in front, except where the sucker-like foot passes through them; the gills are long and narrow, and extend into the siphonal tube; and the two very long siphons are united almost throughout their length. It is also interesting to note that in these animals the rectum does not pass through the heart, as it does in nearly all molluscs, and that a pair of horny or calcareous 'styles' or 'pallets' project from the place where the two siphonal tubes begin to diverge.