192 ZOOLOGY 



the body surfaces continually up and the other down. This is 

 a distinct advance in organization and accompanies a more 

 active life. The Polyzoa are attached in adult life and have 

 lost this symmetry, and many of the Rotifers, while having 

 definite anterior and posterior ends, have lost their right-left 

 symmetry in part, but their embryonic stages are in many 

 respects similar to the more typical forms. By saying that 

 these animals are unsegmented it is meant that in a distinct 

 individual there is usually not a linear series of .equivalent body- 

 parts or metameres. There are however several types which 

 reproduce new individuals by transverse division ("fission"). 

 These new individuals may remain together, temporarily at 

 least, in a chain, as in Microstomum (Fig. 91) or the tape-worm 

 (Fig. 93), forming a strobila. In this condition there is a repeti- 

 tion of all the essential organs in each of the ' 'segments." Some 

 authors regard this process of strobilation as the condition from 

 which the ordinary segmentation, as seen in the Annelida, has 

 arisen, by the adhesion and gradual dififerentiation and coor- 

 dination of the originally similar individuals. 



The animals of these groups agree in the fact that the third 

 or mesodermal layer of tissue becomes established. They are 

 therefore triploblastic animals. In addition to this the meso- 

 derm often, though not universally, splits, forming a caslom or 

 body cavity (§58) wholly separate from the digestive tract. 

 The coelom is lined with mesoderm. All the animal phyla 

 above the Ccelenterates possess this character and on this 

 account are called Ccelomata. 



These animals further agree with those above them in the 

 scale of development in possessing a system of excretory tubules 

 which connect the ccelom, or the mesodermal tissue if there 

 is no coelom, with the outside world. This system eliminates 

 nitrogenous wastes. 



234. Laboratory Exercises. — ^An extended laboratory study of these groups is 

 not necessary in an elementary course, yet enough material representing the 

 various included phyla should be examined to enable the student to justify the 

 separation of these uncertain forms from the more exactly defined phyla, and to 

 show him how ill-defined is the assemblage which we have thus brought together. 

 The Tapeworm of man may sometimes be secured from physicians, and other 



