166 



COLLEGE ZOOLOGY 



described later is quite striking. This has 

 led to the theory that the rotifers are ani- 

 mals somewhat closely related to the ances- 

 tors of the mollusks, annelids, and certain 

 other groups. However, some of the most 

 competent investigators believe that these 

 resemblances of certain trochophore larvae 

 are purely coincidental, the result of adapta- 

 tive radiation and of no evolutionary signifi- 

 cance. It appears more likely that the rotifers 

 have originated from a primitive turbel- 

 larian. 



Some common rotifers are Epiphanes 

 senta (formerly called Hydatina senta), a 

 species used widely for experimental pur- 

 poses; Asplanchna, which often occurs in 

 enormous numbers in the plankton of the 

 Great Lakes; Floscularia, which lives in a 

 transparent tube and has a beautiful corona 

 with 5 knobbed lobes; Melicerta which 

 builds for itself a tube of spherical pellets; 

 and Philodina (Fig. 87), characterized by a 

 slender rose-colored body. 



The rotifers eat microscopic organisms 

 which they convert into their own tissues. 

 In turn the rotifers may serve as food for 

 larger species and eventually, through fish, 

 serve as food for man. Thus the rotifers may 

 serve an important part in a fresh-water food 

 chain. 



PHYLUM BRYOZOA 

 (POLYZOA) 



The Bryozoa, a name that means "moss 

 animals," are so-called because they appear 

 plantlike. They are mostly colonial, and 

 resemble hydroids in form, but they are 

 more advanced in internal structure (Fig. 

 88). The majority of them live in the sea, 

 but a few inhabit fresh water. Bugula is a 

 common marine genus, and Plumatella is 

 the most common fresh-water genus. A col- 

 ony of Plumatella is made up of cylindrical, 

 more or less branched, tubes. These tubes 

 protect the soft parts of the body. The an- 

 terior end of the body of Plumatella consists 



of a rounded ridge called a lophophore; this 

 bears a horseshoe-shaped double row of 

 tentacles. These tentacles are from 40 to 60 

 in number, hollow, and ciliated. When 

 these tentacles are spread out in the water, 

 the cilia cause currents that sweep micro- 

 scopic food organisms into the mouth. The 

 mouth, esophagus, stomach, cecum, intes- 

 tine and anus of Plumatella are shown in 

 Fig. 88. Between the digestive tract and the 

 body wall is a true coelom which is lined 

 with a peritoneum. There are no respiratory, 

 circulatory, or excretory organs. Bryozoans 

 are hermaphroditic. The larvae of some of 

 them resemble a trochophore (Fig. 107). 

 This suggests an ancient origin from some 

 annelid stock. 



Certain fresh-water Bryozoa produce disk- 

 like buds (Fig. 88) which secrete a hard 

 chitinous shell and are known as statoblasts. 

 These survive when the animal dies in the 

 fall or during a drought, giving rise to a new 

 colony in the spring or when the wet season 

 returns. 



Of special interest is the fouling of pipes 

 by certain fresh-water bryozoans. They form 

 thick crusts inside pipes, and dead colonies 

 sometimes break loose, become fragmented, 

 and clog small pipes and meters. 



In prehistoric times there were many more 

 species than are living today. Since their 

 first appearance in the Cambrian period (an 

 early geologic period ) , bryozoans have made 

 substantial contributions to layers of cal- 

 careous rock in every geologic period. 



PHYLUM BRACHIOPODA 

 (LAMP SHELLS) 



The Brachiopoda are marine animals liv- 

 ing within a calcareous bivalve shell (Fig. 

 89). They are usually attached to some ob- 

 ject by a muscular stalk called the peduncle. 

 Because of their shell, they were long re- 

 garded as mollusks. The valves of the shell, 

 however, are dorsal and ventral instead of 

 lateral as in the bivalve mollusks. The name 



