THE MOVEMENTS OF SHORE ANIMALS 169 



are frequently picked up on the sea-shore. Here the action 

 of the sponge seems almost certainly a solvent one, but 

 attention has been drawn to the fact that contractile tissue 

 is particularly abundant in the Clionidae and that the needle- 

 like spicules would have considerable penetrative power. 



(b) Wood-boring Forms. — From the evolutional stand- 

 point the habit of wood-boring in marine animals is of 

 comparatively recent acquisition, standing, as it does, in close 

 relation to the work of man. Since, apart from ships, the 

 majority of structures utilised by these forms occur between 

 tide-marks, they form in consequence a particularly interesting 

 section of our study. 



Here, again, the form of most importance is a bivalve 

 mollusc, the so-called Ship-worm (Teredo). SaUnity appears 

 to be of considerable importance in the distribution of this 

 form, the requirements varying according to the species. 

 For instance, reference has been made elsewhere to an 

 unusual extension of the Ship-worm, Teredo diegemis, in 

 San Francisco Bay, owing to seasons of drought affecting the 

 amount of fresh water entering the Bay ; T. navalts also 

 appears to be intolerant of brackish water and the great 

 outbreaks of this species in Holland in 1730-32, 1770, 1827, 

 and 1858-59 are ascribed to reduced rainfall leading to 

 unusually high salinity in the Zuyder Zee and coastal waters 

 (Caiman, op. cit.). According to the same writer, however, 

 certain tropical species are said to live in perfectly fresh 

 water. Generally speaking, " ship-worms may attack 

 timber at least as high as midway between tide-marks " 

 (see Plate XH). We base our description of the mechanism 

 of boring on that of Caiman. 



Boring is performed by the two very much reduced 

 valves situated at the apex of the long worm-like body. 

 The front edge of these shells is deeply notched, and parallel 

 with the edges of this notch the outer surface of the shell 

 is engraved with a series of fine ridges and grooves, the 

 former of which, under the microscope, are seen to be rows 

 of fine, sharp-pointed teeth, resembling those of a rasp or 

 file. As the shell grows, new rows of teeth are continually 



