190 GENERAL ZOOLOGY 



winter eggs, which are fertilized, have thick shells, are 

 able to pass through a long resting period before hatching, 

 and always develop into females. 



The ordinary life of a rotifer is about six weeks but certain 

 species, if dried slowly, will form gelatinous cysts and are 

 then able to live in a dormant condition for a long time. 

 When encysted, rotifers can stand great extremes of tem- 

 perature without being killed. 



Rotifers in their adult condition resemble a trochophore 

 larva, which occurs in the development of certain molluscs, 

 annelids, and in some other animals. This has led some 

 zoologists to believe that rotifers are closely related to the 

 ancestral forms of molluscs, worms, etc., but such relation- 

 ship cannot be very close because the rotifers are a very 

 specialized group, and would not, therefore, give rise to 

 new lines of descent. 



PHYLUM BRACHIOPODA 



Brachiopods are marine animals having a calcareous 

 bivalve shell and usually a short stalk which fastens them 

 to the ocean bottom. They resemble clams somewhat in 

 appearance but have no close relationship. The valves 

 of a brachiopod's shell are dorsal and ventral, instead of 

 right and left as in clams. The body is very small and the 

 space within the shell is mostly filled by two ciliated arms 

 which create currents for procuring food. Within the body 

 there is a spacious coelom, a stomach, heart, and digestive 

 gland. An anus may be present or absent. All the struc- 

 tures mentioned show some resemblance to those of other 

 worm-like forms, but are modified to suit a sessile life. 



Brachiopods lead a sluggish existence at the bottom of 

 the ocean, and have done so since the earliest geological 

 times. Most of the shells which appear as fossils are the 

 remains of these animals. The abundance of such deposits 

 indicates the past dominance of the Brachiopoda, which 

 at one time shared the earth with trilobites and archaic 



