TUNICATA. 471 



The Molluscous province may thus be primarily divided into Acephala 

 and Encephala. 



The acephalous Mollusca are all aquatic, and are divided into classes 

 according to the modifications of their integument or of their gills. 



The Tnnicata are those which are inclosed bv an elastic gelatinous 

 uncalcified integument ; they breathe either by a vascular pharj'ngeal 

 sac, or by a riband-shaped gill stretched across the common visceral 

 cavity. 



Hunter, who had anatomised the typical forms of this class, and had 

 recognised the homology of their flexible case to the shells of the 

 bivalves, to -which mollusks he saw that Banks's " Dagyza " and the 

 "Squirters" of our own shores were most nearly allied, grouped to- 

 gether the SalpcB and Ascidice, as they are now called, into a natural 

 family, which he termed "soft-shelled;"* this familyjs the same as 

 that afterwards defined and called " shell-less Acephala " by Cuvier, 

 and Ttinicata by Lamarck. All the other Acephala have a shell. 



The Brachiopoda are defended by a bivalve shell, have two long 

 spiral arms developed from the sides of the mouth, and respire by 

 means of their vascular integument or mantle. One valve of the 

 shell is applied to the back, the other to the belly of the animal, 

 which is attached by its shell or a pedicle to some foreign body. 



The Lamellibranchia are bivalve conchiferous Mollusca, which 

 respire by gills in the form of vascular plates of membrane attached 

 to the mantle. One valve is applied to the right side, the other to 

 the left side, of the animal. The common oyster and mussel are 

 examples of this best known class of Acephalous Mollusca. 



The Encephalous Mollusca are divided into classes according to 

 the modifications of the locomotive organs. 



The Pteropoda swim by two wing-like muscular expansions ex- 

 tended outwards from the sides of the head. 



The Gasteropoda creep by means of a muscular disc attached to a 

 greater or less extent of the under part of the body. 



The Cephalopoda have all or part of their locomotive organs 

 attached to the head, generally in the form of muscular arms or 

 tentacula. 



In the last class only do we find, in the present series of animals, 

 an internal skeleton, combined, in some, with a shell. In the rest of 

 the Mollusca the hard parts, when present, are external ; but the 

 integument is sometimes uncalcified and flexible, as in the low organ- 

 ised class which will occupy our attention to-day, and which in this 

 condition of their exo-skeleton afford the parallel to the cartilaginous 

 state of the endo-skeleton in some of the lowest of the vertebrate series. 



* X. vol. i. p. 266. 



H H 4 



