1 84 ZOOLOGY. 



For the first time is found an assemblage of multi- 

 cellular animals whose individuals move with one end con- 

 tinually 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 at- 

 tached 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 em- 

 bryonic stages of these are in many respects similar to the 

 more typical forms. By saying that these animals are un- 

 segmented 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 in- 

 dividuals by transverse division ("fission"). These new 

 individuals may remain together, temporarily at least, in a 

 chain, as in Microstomum (Fig. 89) or the tape-worm (Fig. 

 91), 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 con- 

 dition from which the ordinary segmentation, as seen in the 

 Annulata, has arisen, by the adhesion and gradual differen- 

 tiation 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 

 Coelenterates. In addition to this the mesoderm often, though 

 not universally, splits, forming a coelom or body cavity 

 (56) wholly separate from the digestive tract. The ccelom 

 is lined with mesoderm. All the animal phyla above the 

 Coelenterates possess this character in some measure and on 

 this account are called coclomata. These animals further agree 

 with those above them in the scale of development in possess- 

 ing a system of excretory tubules which connect the ccelom, 

 or the mesodermal tissue if there is no ccelom, with the out- 

 side world. This is sometimes spoken of as the " water- 

 vascular " system to distinguish it from the blood vessels. 



