UNSEGMENTED WORMS IQI 



animals whose individuals move with one end continually 

 foremost and one of the body surfaces continually up and 

 the other down. This is a distinct gain 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 the embryonic 

 stages of these 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 not usually 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), form- 

 ing a strobila. In this condition there is a repetition 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 Annulata, has arisen, 

 by the adhesion and gradual differentiation and coordination 

 of the originally similar individuals. 



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

 or mesodermal layer of tissue becomes more important than 

 it is among the Crelenterates. They are therefore triploblastic 

 animals. In addition to this the mesoderm often, though 

 not universally, splits, forming a ccelom or body cavity (58) 

 wholly separate from the digestive tract. The ccelom is lined 

 with mesoderm. All the animal phyla above the Ccelenterates 

 possess this character in some measure 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 ccelom, with the outside world. This system is believed 

 to get rid of nitrogenous wastes. 



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

 not desirable, yet the teacher should secure enough material representing the 

 various included phyla to enable the student to justify the separation of these 



