SABELLA. 127 



of the animal membrane growing round a slight 

 projection in the siliceous pebble might be a suffi- 

 cient bond. 



The Trochus concliyliophorus derives the same 

 sort of protection from foreign substances, being 

 covered however with entire stones and shells, in- 

 stead of pulverized or broken pieces of them. In 

 this case no one doubts the fact of its being a true 

 Trochus, because the foundation of the aggregate 

 is regularly spiral, and possesses the characteristic 

 aperture. Why then should Sabella be expelled 

 the order, as it often has been, because the sheath 

 is not spiral, but straight and tubular ? Some few 

 species may have found admittance, which do not 

 perhaps belong even to the class of Vermes, as the 

 larvae of some insects are known to provide them- 

 selves with a very similar receptacle to that of a 

 real Sabella. There is one never-failing mark of 

 genuineness in most perfect specimens of this ge- 

 nus, — that of the end being, as it were, fringed, or 

 produced into numerous ragged processes of the 

 same nature with the shell. They are, however, 

 so extremely friable, that perfect specimens are 



