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 a number of ciliated tentacles, 

 called the lophophore (Fig. 32-20). The di- 

 gestive tract is V-shaped, so that the anal 

 opening discharges the fecal wastes outside 

 the protective envelope in the region border- 

 ing the lophophore (Fig. 32-20). There is a 

 nerve ganglion situated between mouth and 

 anus, but no circulatory, respiratory, or ex- 

 cretory organs are present. 



Fossil remnants of the Bryozoa are widely 

 distributed in geological strata extending 

 back to the Ordovician period; and many 



