214 



ZOOLOGY. 



articulates, interpolated between the penultimate and ter- 

 minal segments of the body. At D, the body is many- 

 jointed, the tentacles well developed, the large temporary 

 bristles have been discarded, and the worm can be identified 

 as a young Polydora. 



It is probable that Polydora is hatched as a trochosphere 

 like that of Polyzoa, Brachiopoda and certain mollusks. 

 The young Terebrellides Ktroemii, and of Lumbriconereis, 

 are at first trochospheres, i. e., the free-swimming; 

 germ is spherical, with a zone of cilia, two eye- 

 spots, and no bristles. Thus the earliest stages of 

 Polyzoa, Brachiopoda, Lamellilranchiata, Gastro- 

 jioda, and even of a Cephalopod (Fig. 215), Nemer- 

 tina, and Annelides are almost identical. Farther 

 ^ a ] on g in their developmental history, the cepha- 

 Af|r A. lg. lula of the Annelides (Fig 147, A, B, and 149),. 

 is like that of certain Echmoderms (Fig. 149),, 

 GcpJtyrea, Polyzoa, Brachiopoda, and Mollusca. It may 

 here be observed that the free-swimming larvae of these types 

 of invertebrate animals are the young of more or less seden- 



D 



Fi<j. 149. Cephalula stage of Echmoderms and Worms, lateral view. A, Holo- 

 thurian, 7?. Star-fish, O, D, of Anuelides. 



o, mouth ; i, stomach ; a, vent ; , prseoral ciliated band, in B, C, D, independent ; 

 in .1 surrounding an oral region. From Gegenbaur. 



tary parents. In this way the species becomes widely dis- 

 tributed through the action of the marine currents, and too 

 close in-and-in breeding is prevented. 



Certain Annelides sometimes multiply by self -division, the- 

 process being called stroMlation. This is commonly observed 



