114 LIFE BY THE SEASHORE. 



Their presence may often be demonstrated even when they 

 cannot be seen, by drawing a needle over the hook-bearing 

 area, when a slight grating sound will be heard. Unlike 

 the dorsal bristles these ventral hooks occur throughout the 

 body, except on the extreme anterior segments. In the 

 anterior region in the living worm it will be found that 

 the ventral surface is very bright red in colour, and glandular 

 looking. This is due to the presence of fourteen to twenty 

 pairs of " gland-shields," which secrete the mucus which is 

 the basis of the tube. 



A considerable number of Terebellids live on the shore, 

 differing from one another chiefly in the structure of the 

 gills, the number of dorsal lobes, of gland-shields, etc. 

 Small specimens of Terebella or of others will live for a 

 time in confinement, when the process of tube-building can 

 be watched. The worms may sometimes be induced to 

 build an incomplete tube along the side of the aquarium, 

 so that the worm may be watched through the glass within 

 its tube. Like other tube-inhabiting forms, Terebella con- 

 cliilega shows considerable power of adapting its "masonry" 

 to the special conditions in which it may be placed ; thus 

 specimens living in deep water construct tubes which in 

 several respects differ from those of shallow-water forms. 



Another interesting tube-worm, smaller and less abundant 

 than Terebella, is Peciinaria belgica (see Fig. 38), which is 

 to be found in sandy pools. Its tube is short, usually about 

 one and a half inches, is without a fringe, but displays a 

 neatness of workmanship which makes the tubes of Terebella 

 seem coarse and clumsy. It is constructed of sand grains, 

 which are all of the same size, and are smoothly and evenly 

 worked into a plaster of mucus, so as to form a beautiful 

 mosaic. The tube is firm enough not to collapse when the 

 tenant is removed, and is open at both ends. The large end 

 corresponds to the head of the worm, but it is this end which 

 in life is buried in the sand, the narrower posterior end pro- 

 jecting from the surface of the sand. The worms live well 

 in captivity, and the habits may be readily observed in 

 specimens placed in a glass jar with clean water and a layer 

 of sand. In such specimens you should see a beautiful 

 crown of golden bristles (b in Fig. 38) protruded from 

 the large end of the tube, and used as a trowel to excavate 



