210 MR. R. OWEN ON THE ANATOMY OF THE CALYPTRAID. 
porary branchie of the foetal Plagiostomous Fishes, each filament of which also contains 
a single artery and vein. 
The nervous system consists of five ganglia: four disposed round the esophagus at 
the lower part of the neck, and one small one at the internal angle of the branchial 
aperture. The two superior oesophageal ganglia are the smallest: they give off the 
nerves of the tentacles without the interposition of another ganglion ; they also give off 
lateral filaments to the cervical aliform expansions. The two larger subcesophageal gan- 
glia give off the nerves of the foot and viscera, and from the left of them a nerve extends 
to the entrance of the branchial chamber, where a small ganglion sends a nervous twig 
along the floor of that cavity. 
In the cabinets of the Naturalist, the shells of the Crepidule and Calyptree attract by 
the singularity rather than the beauty of their forms ; but they are still more interest- 
ing as manifesting some of the successive stages of complexity in the passage from the 
simple Patella to the spiral univalve. 
The superaddition of the internal plate or cup is obviously immediately caused by 
the dorsal fold or duplicature of the mantle, the margins of which, being endowed with 
the same power of secreting shell as the exterior margin itself, form the internal plate 
or cup according to the extent of the duplicature. The necessity for such a superaddi- 
tion is probably to be sought for in the more active locomotive powers of Calyptrea as 
compared with Patella ; the foot in the former, being from its organization adapted to 
more extensive and frequent contractions, would be liable to affect the superimposed 
viscera if they were in immediate contact with it. A calcareous plate, the first stage of 
a columella, is therefore interposed, which supports the viscera, and separates them from 
the locomotive organ. 
As respiration has a direct relation to locomotion, so we find the Calyptreide ap- 
proaching the higher marine univalves in the structure and position of the part dedi- 
cated to this function. The branchial filaments are, however, arranged in a single 
series ; and the entry to the branchial chamber is not prolonged into a siphon, as in 
Buccinum and the higher Pectinibranchiata, which in their double branchie and respi- 
ratory siphon approximate more closely to the Cephalopods. But throughout the family 
of Calyptreide I have found the extent of the respiratory /amina to be in direct relation 
with the extent of the internal shell, and the superior extent and organization of the 
foot. 
