BIVALVES AND CUTTLES 285 



dered less resistant to wave-action, and are torn off from 

 the solid mass of rock. They are then often utilised in 

 rockeries, and on the East Coast it is common to find these 

 built of such honeycombed stones still retaining the little 

 shells. In the living condition the animal is very common 

 on the shore, where its bright red siphons are to be seen 

 protruding from rock surfaces. When touched they eject 

 a forcible jet of water and then disappear. By breaking 

 the rock, specimens can be obtained without difficulty, the 

 shells though small being solid and not readily broken. 

 They are oval in shape and gape at the posterior end ; the 

 colour is white, and the shells generally without much 

 beauty. It seems unnecessary to describe the details, as 

 the bright red siphons and the habitat form very distinctive 

 characters. 



Even more interesting are the species of Pholas, which 

 have singularly beautiful shells. One species, P. crispafa, 

 is exceedingly common in the Firth of Forth, where it 

 excavates the shale in all directions. Some of its charac- 

 ters have already been noticed in Chapter I., so we may 

 confine ourselves here to some details of the shell. As in 

 all species, this gapes widely both in front and behind; there 

 is no ligament nor teeth, but there is an accessory valve, or 

 dorsal shield, beside the hinge; the hinge-plate is reflected 

 over the beaks, and the shell is divided into nearly equal 

 parts by a broad oblique furrow. Of the two regions so 

 formed the anterior is furnished with about twenty rows 

 of overlapping prickles, supposed to be of great importance 

 in boring ; the posterior region is quite small. The whole 

 shell is pure white and very brittle, so that a little care is 

 necessary to obtain uninjured specimens. Another species, 

 P. Candida, also occurs in shale, but may be distinguished 

 by the absence of the furrow, the prickles covering the 

 whole surface except a space at each end. 



We have now mentioned most of the Bivalves likely to 

 be found living between tide-marks. Of the Mollusca there 

 still remain the Cephalopoda, or Cuttles, specialised forms 

 in which the foot has grown up round the head and become 

 split into eight or ten sucker-bearing arms. Other character- 

 istic structures are the funnel, through which jets of water 

 can be ejected, thus producing motion, and the ink-bag, 



