MOLLUSCGA. 
This group includes the shell-bearers, as well as the naked forms 
whose shells are either hidden under the mantle, or are entirely 
absent. Cuvier was the first to place them among the principal 
groups of the Animal Kingdom, and in the following order :— 
1. The Cephalopoda; 2. The Gastropoda (Snails and Slugs, 
terrestrial and aquatic) ; 3. The Pelecypoda ; 4. The Brachiopoda ; 
5. The Ascidians (Sea-Squirts) ; 6. The Pteropoda (Winged 
Snails), Clio, &c. ; 7. The Cirrhipoda (Barnacles). This last soon 
gave way to malacological criticism. The Ascidians, which Cuvier 
considered to be Pelecypoda without shells, soon followed, after 
being tossed about and joined to various groups, even with the 
Vertebrata, through embryonic similarities. Instead of sceret- 
ing a calcareous shell on the surface of their bodies they envelope 
themselves in a case varying in consistence from soft jelly to tough 
leather. Tle Cephalopoda derive their name from the attachment 
of the feet, or principal locomotive organs, to the head ; in their 
general conformation the molluscan character is retained. There 
is no trace of the muscular foot of the Gastropod, except in the 
Nautilus, which in its general structure approaches that order most 
nearly ; it is the only existing Cephalopod which is wholly provided 
with an external shell. From the earliest geological periods 
thousands of species of this Order swarmed in the primeval 
seas, The Cuttle-fish, which has no external shell, is furnished 
with an ink-bag, from which it can discharge a _ black 
pigment, and easily effect a safe retreat from the enemy. 
Sir Richard Owen divided the Order into two Sections, Dibran- 
chiata and Tetrabranchiata, founded on tke existence of two sym- 
metrical gills in one and four in the other. The Octopus, the 
Sepia, the Loligo, and the Argonaut belong to the first section, 
