Polyzoa. 



181 



are the so-called avicularia (Fig. 140 G), small individuals, destitute 

 (or with only rudiments) of tentacles^ mouth, and digestive tract, but 

 with a large movable oper- 



4Sr^,>'^ ' 



5^, '' r>> 



ii/'^cm 



culum, which can open and 

 shut. The best developed 

 avicularia resemble crabs' 

 claws or birds' beaks, for 

 the tip of the operculum 

 is bent like a hook, and 

 bites upon an outgrowth of 

 the body. They seem to 

 be a kind of defensive 

 person, to catch the animals 

 crawling over the surface 

 of the colony. More rare 

 are the vibracula, also 

 small reduced persons, 

 whose operculum is deve- 

 loped into a long whip- 

 like process, which sweeps 

 over the surface of the 

 colony. 



The Polyzoa undergo a 

 metamorphosis. There 

 is a free-swimming larva, 

 whose cilia are either evenly 

 distributed over the body, 

 or restricted to special 

 regions (ciliated ridges or 

 tufts) ; sometimes there is 



a hard cuticle or shell upon part of the body, usually it is entirely 

 naked. 



They are very numerous in all seas ; a few live in fresh water. 

 The fresh-water forms, whicli are found on water plants, etc., generally have a 

 horseshoe-shaped lophophore ; and form a delicate branched colony, which is not 

 raised much above its support : but some species grow erect, neighbouring 

 branches suppoi-ting one another reciprocally, and thus forming large clumps. 

 Amongst the marine forms are the Membraniporidss, which may often be seen 

 forming calcareous incrustations upon the sm-faces of aU large sea-weeds. 



Fig. 142. Fresh-water Polyzoon, bi- 

 sected. Diagrammatic, a anus, e excretory aperture, 

 m mouth, mu muscle, n nerve ganglion, st statoblast 

 on the funiculus. — Orig. 



BracMopoda. 



The body is generally enclosed within two calcareous, or rarely, 

 chitinous shells, somewhat like those of the Lamellibranchs, with which, 

 therefore, the Brachiopoda were in times past associated. As a matter 

 of fact the two groups are in no. wise nearly related to one another. 



