MOLLUSCOIDA : POLTZOA. 239 



extending from the testis to the stomach upon which they 

 may usually be seen in different stages of growth. They do 

 not appear to be set free from the perigastric space prior to 

 the death of the adult, and when liberated, they are enabled 

 to float near the surface of the water, in consequence of the 

 cells of the marginal ring, or ' annulus,' being spongy and 

 filled with air. They must be looked upon as ' gemmce 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 Phylactolcemata, distinguished by 

 the possession of a bilateral horse-shoe-shaped lophophore, and 

 of an ' epistome ' arching over the mouth ; and the Gymnolce- 

 inata, in which the lophophore is orbicular, and there is no 

 epistome. 



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



ORDER I. PHYLACTOLCEMATA. 



Lophophore bilateral ; mouth with an epistome. 



Sub-order 1. 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.EMATA. 



Lophophore orbicular, or nearly so; no epistome. 



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



Polypide completely retractile ; evagination of tentacular sheath 

 imperfect ; consistence horn} 7 or sub-calcareous. 



Sub-order 4. Chellostomata (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 sphinc- 

 ter ; cells not tubular ; consistence calcareous, horny, or fleshy. 



Sub-order 5. Cyclostomata (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. 



