114 
SAB ELLA. 
(Plate XXII. Fig. 5.) 
Shell tubular, formed of sandy and calcareous 
particles agglutinated and inserted in a membra- 
naceous sheath. 
The best claim which the Sabella could assert 
to be ranked among Testacea is, that the cement 
with which the fragments of organised and siliceous 
matter are fastened to the mould is really calca- 
reous, and not an animal gluten only. By immers- 
ing a portion of the shell in muriatic acid the cal- 
careous particles will be dissolved, and many, if not 
all the indissoluble fragments will be precipitated, 
leaving the membranaceous sheath entire, and im- 
pressed with the forms of the substances which were 
attached to its exterior surface. Still, some will be 
found to adhere and resist the action of the acid : it 
is therefore difficult to affirm whether they have been 
fastened by a gelatinous secretion, or by a cal- 
careous one. In either case, the fibre of the animal 
membrane growing round a slight projection in the 
siliceous pebble might be a sufficient bond. 
