CHAP. II. TESTACEOUS MOLLUSKS GENERALLY. 27 



tribes are at once known, either by being covered with 

 a hard shell, or, if without this protection, by having 

 white blood, and being destitute of any processes which 

 perform the office of feet. Many, indeed, are fixed to 

 the surface of rocks, and other substances, where they 

 remain during life ; while others bore for themselves 

 internal cavities, or cells, in which they take up their per- 

 manent habitation; while the least organised are parasitic. 

 The similarity of some of the naked Testacea to the^rme- 

 lides, or sea-worms, is so great, that the older naturalists 

 classed them together ; and even to this day we find a 

 whole division mixed up with the annulose Vermes, or 

 true intestinal worms. Let the student, however, bear 

 in mind that the animals of which we are now speaking 

 have neither joints to their bodies or limbs, nor any pro- 

 cesses, by which they can crawl ; and that their blood 

 is white, and not, as in the Annelides, red. Further- 

 more, the testaceous Mollusca never assume the shape 

 of the polypes ; nor do they possess that radiated form 

 and arrangement of filaments, which give such a pecu- 

 liar aspect to the Radiata. By these circumstances, 

 therefore, the observer may discriminate, in ordinary 

 cases, the characteristic marks of even the aberrant or 

 less perfect Testacea ; while he can be in no doubt as 

 to those which constitute the types. The perfection of 

 the class is seen in those animals whose body is pro- 

 tected by a hard calcareous covering, or shell, either in 

 the form of a twisted or convoluted cone, variously 

 modified, or composed of two principal valves or 

 pieces, more or less flattened, and united by a cartila- 

 ginous hinge. These shells are generally ornamented 

 with a variety of colours, and are frequently polished, 

 on their external surface, in the most beautiful and 

 perfect manner, by the animals themselves. The single 

 shells are called Univalves, and such is the periwinkle 

 and snail ; while the double are termed Bivalves, and 

 are exemplified in the oyster and the cockle. As 

 nature, however, proceeds from one to the other of 

 these groups by gradations of structure, we consequently 



