TUNICATA, BRACHIOPODA. 269 



mouth is simply a pharynx or beginning of the oesophagus without 

 jaws, tongue, or mouth properly so called. Such MoUusca are 

 termed Acephala. All other Mollusca are provided with a head, 

 which generally supports feelers or soft tentacula, eyes, and a mouth 

 armed with jaws. 



The acephalous Mollusca are all aquatic, and are divided into 

 classes according to the modifications of their integument or of 

 their gills. 



The Tunicata are those which are inclosed by an elastic gela- 

 tinous uncalcified integument ; they breathe either by a vascular 

 pharyngeal sac, or by a riband-shaped gill stretched across the com- 

 mon visceral cavity. 



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. 



The Lamellibranchia are bivalve conchiferous Mollusca, which 

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

 to the mantle. 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 an undivided 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 at- 

 tached to the head, generally in the form of muscular arms or ten- 

 tacula : in this class only do we find, in the present series of animals, 

 an internal skeleton. In the rest of the Mollusca the hard parts are ex- 

 ternal ; but the integument is sometimes uncalcified and flexible, as in 

 the low organised 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. 



To connect the Tunicata with any of the classes of animals which 

 we have previously considered, it is necessary to revert to the Polypi, 

 for it is in this group of the Radiata that we shall find the animals 

 which have the closest natural alliance with the present class of 

 Mollusca. 



Suppose a Bryozoon to have its ciliated oral tentacula reduced to 

 mere rudiments, and to have the pharynx enormously expanded, with 

 its vascular internal surface richly beset with vibratile cilia ; it would 

 then be converted into an Ascidian, and the transition from the 



