234 ON THE CLASSES OF THE 



we have noticed in the P kolas become here more deve- 

 loped, and unite so as to form a tube. In some cases, 

 as in the genus A&pergillum, the bivalve shell confounds 

 itself with this tube, which has in a manner the true shells 

 inlaid into its substance. 



These disguises, and this variety of change in natural 

 forms, sadly distressed naturalists until they began to 

 study the animals which inhabited the shells, as well as 

 their habitations. The discovery of a bivalve shell in- 

 closed in a testaceous tube was indeed a fact well cal- 

 culated to excite surprise; and still more extraordinary 

 must it have been to see the shell inlaid in the side of the 

 tube, and forming part of it. We owe to M. Lamarck 

 the explanation of these truths, a little attention to which 

 may, as I conceive, also serve to show us how nature 

 passes in the Mollusca from the bivalve animals to the 

 univalve. 



The genus Bulla cannot strictly be said to possess 

 tentacula, or even head. On this account Cuvier has 

 very happily named the family to which it belongs Acera, 

 inserting it in a group, the rest of which are all provided 

 with tentacula. But this deficiency of the principal organs 

 of sense in a Bulla, its branchiae covered by the mantle, 

 the simple nervous system and the voluminous liver, em- 

 bracing closely the several convolutions of the intestinal 

 canal, are all properties which we have seen to belong to 

 the Acephala; and were it not that these last have bivalve 

 shells, and the other is a univalve, naturalists would no 

 doubt have adopted some method of connecting them in 

 their various systems. The nervous system of the Acera 

 consists of two ganglions situated at the sides of the oeso- 

 phagus, and united by a collar of the same nature, which 



