" SHELL-FISH." 2O5 



population. The shell has a distorted look about it, and the 

 valves will not fit properly, the ends gaping to allow the foot 

 and the syphons free play. It is very variable, however, and 

 consequently has been a splendid subject for the variety- 

 mongers and species-splitters, who have manufactured quite a 

 long list of species and genera out of it. It changes a good 

 deal at different periods of its life, and thus affords opportuni- 

 ties for careful descriptions made from isolated specimens 

 utterly disagreeing with each other ; therefore, the individuals 

 described must belong to different species ! In its early state 

 the shell is symmetrical, and has two minute teeth in each 

 valve ; but before it has reached maturity it has lost its claim 

 to be considered graceful or symmetrical, and has either worn 

 its milk-teeth out or abandoned them as useless. The shell is 

 covered with ridges and wrinkles, and it is by their help that 

 it carves out its chamber in the rock. Sometimes on turning 

 aside a curtain of weeds from a rock-face you will see a large 

 number of crimson points, which, however, instantly disappear 

 if they have been ever so lightly touched by the weeds. These 

 are the ends of the borers' syphons, protruded from their 

 ventilation holes ; they are united almost to their extremities, 

 and present the appearance shown in our figure. 



The Piddock, or Finger Pholas (Pholas dactylus), is a much 

 larger species with some difference of structure. Its pure 

 white shell, though thin and fragile, is covered in front with 

 rasp-like ridges, and by means of it the chambers and tunnels 

 of the rock are bored. Holding to the rock with the clear 

 crystalline foot, the Pholas gives its shell a swing half-way 

 round in one direction, then a swing back, and so by alternate 

 half-revolutions, the rasping of the shell gradually excavates 

 a chamber sufficiently large to shelter it, communication with 

 the outer world being maintained by the large syphons. So 

 far there is no very great difference between the Pholas and 

 the Saxicava ; but the Pholas is peculiar in that it possesses 

 neither ligament nor hinge, and in addition to the orthodox 



