. Principles of modern biology. Biology. 642 - Herediiy and Evolution MOUTH. GANGLION PROBOSCIS STYLET DIGESTIVE TRACT BLOOD VESSEL ANUS Fig. 32-18. A nemertine worm; diagram of essential structures. streams and ponds, or even in wayside pud- dles. The rotifers, nevertheless, are multicellu- lar, bilaterally symmetrical, triploblastic ani- mals, which possess a complete tubular di- gestive tract (Fig. 32-19). Also there is a body cavity in the mesoderm. However, this cavity is called a pseudocoel, rather than a coelom, since it does not possess any definite epithelial lining, or peritoneum (p.
. Principles of modern biology. Biology. 642 - Herediiy and Evolution MOUTH. GANGLION PROBOSCIS STYLET DIGESTIVE TRACT BLOOD VESSEL ANUS Fig. 32-18. A nemertine worm; diagram of essential structures. streams and ponds, or even in wayside pud- dles. The rotifers, nevertheless, are multicellu- lar, bilaterally symmetrical, triploblastic ani- mals, which possess a complete tubular di- gestive tract (Fig. 32-19). Also there is a body cavity in the mesoderm. However, this cavity is called a pseudocoel, rather than a coelom, since it does not possess any definite epithelial lining, or peritoneum (p. 298). Characteristi- cally, the head end of the cylindrical body bears two circlets of active cilia, which tend to resemble whirling microscopic wheels. The tapered tail usually terminates by forking into two sharply pointed (frequently sticky) toes, by which the animal can attach itself to the substratum (Fig. 32-19). Of the 12,000 or so known Rotifera, al- most all are free-living, solitary species, in- habiting fresh water. However, a few live in salt water and a few are parasitic. No fossilized Rotifera have been found, prob- ably because they do not form any mineral- ized supporting or encasing structures and because they are so very small. The Bryozoa. These so-called "moss ani- mals" grow in colonies of a size and shape that may have a rough resemblance to clumps of mosses. Encrusting a submerged rock or pile in shallow parts of the sea, they may also look like colonial Hydrozoa (p. 632). Bryo- zoan colonies, like those of the Hydrozoa, are usually covered by a chitinous or calcare- ous envelope. Structurally, however, the individuals of a bryozoan colony are more complex (Fig. 32-20). Each is a triploblastic, bilaterally sym- metrical animal. Also each possesses a com- plete tubular digestive tract and a true coe- lom, which is definitely delimited by a peri- toneal lining. Characteristically, the mouth is surrounded by a horseshoe-shaped struc- ture, bearing
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