ANIMAL KINGDOM. 235 



surrounds the canal and is in fact the brain. From each 

 of these two cerebral ganglions proceeds a nerve, which on 

 meeting the other forms by the union the visceral gan- 

 glion. It is therefore manifest that, with the exception of 

 the cerebral ganglion of the Acephala being divided here 

 into two separate lobes, there is the strongest affinity be- 

 tween this nervous system and that of the Bivalve Mol- 

 lusca. This affinity however is worthy of further exami- 

 nation. Let Bulla lignaria Linn, be the species under ob- 

 servation : we notice in the first place a fleshy disc, which 

 serves for a foot; this is bisected by a transverse furrow, 

 which extends equally over the back of the animal, and 

 separates the hinder part which envelopes the shell from 

 the anterior part of the body, which is free. This front 

 division of the body, which is completely out of the shell, 

 contains the oesophagus, stomach, brain, and salivary 

 glands. The stomach occupies the greatest part, and is 

 completely protected by three testaceous pieces, which 

 form a sort of prism vulgarly called the gizzard. Two of 

 these three pieces are fiattish and precisely of the same 

 form, so as to present what may be termed a regular bi- 

 valve ; they are united together at the edges by a muscular 

 substance composed of fleshy fibres, and in this fleshy 

 tunic at the upper end is inlaid the third testaceous piece, 

 which is oblong and irregular. This apparatus can be 

 compared to nothing among the Mollusca, unless it be to 

 the two valves of a Pholas with the insulated piece 

 under the hinge. In the Acera it covers a no less impor- 

 tant part of the body than the stomach ; and we have 

 already seen in the genus Teredo that the shell of an Ace- 

 phalous animal may become too small for the body. 



But if all doubt on the subject of a Bulla being the last 



