22 BRITISH 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 remaming 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 
