MOLLUSCOIDA: TUNICATA. 265 



filled with air. They must be looked upon as "gemma pecu- 

 liarly encysted, and destined to remain for a period in a qui- 

 escent or pupa-like state." (Allman.) 



As regards the development of the Polyzoa, the embryo upon 

 its emergence from the ovum presents itself as a ciliated, free- 

 swimming, sac-like body, from which the polypide is subse- 

 quently produced by a process of gemmation. 



DIVISIONS OF THE POLYZOA. The Polyzoa are divided into 

 two divisions or orders the Phylactolczmata (fig. 95, 3), dis- 

 tinguished by the possession of a bilateral horse-shoe-shaped 

 lophophore, and of an " epistome " arching over the mouth ; 

 and the Gymnoltzmata (fig. 95, 2), in which the lophophore is 

 orbicular, and there is no epistome. 



TABLE OF THE DIVISIONS OF THE POLYZOA. (AFTER BUSK.) 

 ORDER I. PHYLACTOL^MATA. 



Lophophore bilateral ; mouth with an epistome. 

 Sub-order I. Lophopea (fresh- water). 



Arms of lophophore free or obsolete ; consistence horny, sub-cal- 

 careous. 

 Sub-order 2. Pedicellinea (marine). 



Arms of lophophore united at their extremities ; consistence soft, 

 fleshy. 

 ORDER II. GYMNOL^MATA. 



Lophophore orbicular, or nearly so ; no epistome. 

 Sub-order 3. Pahidicellea (fresh-water). 



Polypide completely retractile ; evagination of tentacular sheath 

 imperfect ; consistence horny or sub-calcareous. 

 Sub-order 4. Cheilostomata (marine). 



Polypide completely retractile ; evagination perfect ; orifice of cell 

 sub-terminal, of less diameter than the cell, and usually closed with 

 a movable lip or shutter, sometimes by a contractile sphincter ; cells 

 not tubular ; consistence calcareous, horny, or fleshy. 

 Sub-order 5. Cydostomata (marine). 



Cell tubular ; orifice terminal, of the same diameter as the cell, 

 without any movable apparatus for its closure ; consistence cal- 

 careous. 

 Sub-order 6. Ctenostomata (marine). 



Orifice of the cell terminal, furnished with a usually setose fringe 

 for its closure ; cells distinct, arising from a common tube ; con- 

 sistence horny or carnose. 



CHAPTER XLIII. 

 TUNICATA. 



CLASS II. TUNICATA (Ascidwida). r Yte members of this class 

 of the Molluscoida are defined as follows : " Alimentary canal 



