22 BEITISH MARINE TESTACEOUS MOLLUSCA : 



agents for the opening and closure of the valves, in aid of other 

 special muscles. 



It may be said that these views are far-fetched, or imagi- 

 nary; but if naturalists would seriously consider that the oper- 

 culum of the Balani is of the nature of an apophysis for a 

 particular purpose, they can only come to the conclusion that 

 such an appendage can have no other use but for the support, 

 issue, and regulation of the buccal tentacular arms. If it is 

 asked, what is the use of the apophyses of the Palliobranchiata? 

 the same answer only can be given. These subumbonal blades 

 appear in the Pholades and Teredines ; their use is the same 

 as the processes in the Palliobranchiata, and the opercula of 

 the Cirripoda, for the support and regulation of certain organs 

 of the animal. 



With the exception of the genus Hypothyris, the shells of 

 these animals are invariably covered with punctures or fora- 

 mina, which would appear to give them a slight cross with 

 still inferior classes. There is nothing peculiar in the repro- 

 ductive organs ; a similar disposition of them is seen on the 

 mantle of Anomia and in some of the Pectines. In this group 

 the animal is deposited in its shell differently from the Lamel- 

 libranchiata ; the larger or convex valve, though uppermost, is 

 really, with respect to the animal, the dorsal one, and the 

 under valve the ventral one ; an approach to a similar position 

 of the animal may be observed in Anomia, Pecten and Ostrea. 

 In the Palliobranchiata the nervous system is more sparingly 

 developed; they have lost the knotted spinal cord of their 

 remoter relatives, the Cirripoda, which in them is disposed in 

 detached ganglions, the result of the change they have under- 

 gone in the plan of their organization;* and for the same 

 reason, the articulation of the spiral arms has disappeared, 

 and is replaced by their tubular flexibility, the use and form 

 of the organs still remaining the same in them as in the 

 Cirripoda. 



I think enough has been said to show that the pallio- 

 branchiate bivalves are of inferior organic structure to that of 

 the lamellibranchiate Acephala. We refer those who are 

 desirous to be informed of what is known of the organization 



