444 POLYZOA. MARINE FORMS. 



Bmcerbankia (Fig. 704), that is to say, those organs are placed 

 around the mouth in such a way that, when expanded, they 

 form a sort of cup or funnel ; hence the animals of this Order 

 have been called INFUNDIBULATA. The Bowerbankia and 

 Flustra, already referred to, belong to this group, which also in- 

 cludes all the species (such as Eschard) in which the polyzoary 

 is of a calcareous nature. Of the Flustrce, some, like the com- 

 mon Sea-Mats, form free branching leaves ; whilst others, which 

 are more delicate in their appearance, spread themselves in the 

 form of a gauzy membrane upon the fronds of sea-weeds. An- 

 other form is presented by the Alcyonidium, in which the cells 

 are united by a fleshy or spongy mass. In some species of this 

 group we find some curious appendages attached to the poly- 

 zoary. These appendages form forceps, consisting of a larger 

 upper, and a smaller lower horny valve, the latter being move- 

 ably articulated to the upper piece, so that the whole has a strong 

 resemblance to the skull of a bird. Hence they have been de- 

 nominated avicularia, or "Bird's-head organs." They are 

 moveably attached to the polyzoary, sometimes by the interven- 

 tion of a short footstalk, and are constantly vibrating during 

 the life of the animal, and even for some time after its death ; 

 the horny pieces representing the beak opening and closing 

 occasionally at the same time. The office of these singular ap- 

 pendages, in the economy of those Polyzoa in which they occur, 

 is by no means satisfactorily made out ; they have been supposed 

 by some writers to serve for the protection of the colony of which 

 they form a part, by keeping off the attacks of other marine 

 animals ; whilst others have considered them to act as purveyors 

 of food, small animals being often captured by them. Mr. Gosse 

 has recently put forward an opinion, that they may subserve the 

 latter purpose in a secondary manner, by seizing and holding 

 any passing creatures of moderate size, the presence of which 

 would then attract a multitude of Infusoria, which might serve 

 as legitimate nourishment. In some forms the place of these 

 appendages is taken by long stiff rods on the margins of the cells, 

 which are also in constant vibration. 



1079. In the fresh-water Polyzoa, the tentacles, instead of 



