194 MOLLUSCA. 



apparently supplied by a dilatation of the body of the cell 

 itself." (Busk.) 



The " avicularia " and " vibracula " are peculiar appendages 

 of the ectocyst, supposed to be weapons of offence and defence, 

 or to subserve some unknown function in the economy of the 

 colony, and believed by Huxley to be peculiarly modified 

 polypides. The avicularia, or "bird's-head processes," differ 

 a good deal in shape, but consist essentially of a " movable 

 mandible and a cup furnished with a horny beak, with which 

 the point of the mandible is capable of being brought into 

 apposition " (Busk). In shape they are often closely similar 

 to the head of a bird (fig. 131, 3), and they perform a peculiar 

 snapping movement, which is continued long after the apparent 

 death of the colony. In many respects, the avicularia are 

 comparable with the " pedicellariae " of the Sea-urchins and 

 Star-fishes. In the " vibracula," the place of the mandible of 

 the avicularium is taken by a bristle or seta, which is capable of 

 extensive movement. 



The following table exhibits the leading groups of the 

 Polyzoa : 



TABLE OF THE DIVISIONS OF THE POLYZOA. 



ORDER I. PHYLACTOL^EMATA. 



Tentacles arranged in the shape of a horse-shoe or crescent. Mouth 

 furnished with a valve-like organ or "epistome." 



Sub-order I. Lophopea (fresh- water). Arms of the tentacular disc ("lopho- 

 phore") free or obsolete ; consistence, horny or sub -calcareous. 



Sub-order 2. Pedicellinea (marine). Arms of the tentacular disc united 

 at their extremities ; consistence, soft and fleshy. 



Sub-order 3. Rhabdopleurea (marine). Coencecium branched, adherent, 

 membranous, with a chitinous rod on its adherent side. Tentacular disc 

 horse-shoe-shaped. No epistome (?) 



ORDER II. GYMNOL^EMATA. 



Tentacles arranged in the form of a more or less complete circle. No 

 valve-like organ, or "epistome," arching over the mouth. 



Sub-order 4. Paludicellea (fresh-water). Polypide completely retractile ; 

 evagination of tentacular sheath imperfect ; consistence, homy or sub- 

 calcareous. 



Sub-order 5. 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 con- 

 tractile sphincter; cells not tubular ; consistence, calcareous, horny, or fleshy. 



Sub-order 6. Cyclostomala (marine). Cell tubular ; orifice terminal, of 

 the same diameter as the cell, without any movable apparatus for its closure; 

 consistence, calcareous. 



Sub-order 7. Ctenostomata (marine). Orifice of the cell terminal, furnished 

 with a usually setose fringe for its closure ; cells distinct, arising from a 

 common tube ; consistence, horny or carnose. 



