. Text book of zoology. Zoology. 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
. Text book of zoology. Zoology. 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 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 nearly related to one Please note that these
Size: 1290px × 1937px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1896
