234 



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 Stroemii, 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, Lamellibranchiata, Gastro- 

 poda, and even of a Cephalopod (Fig. 220), Nemer- 

 tina, and Annelides are almost identical. Farther 



Fig. 154.- 



,arva of 



Phyllodoce. 



Larva of a ] on g m their developmental history, the cepha- 





A.A g - Ma of the Annelides (Fig. 153, A, B, and 155), 

 is like that of certain Echinoderms (Fig. 155), 

 Gepliyrea, 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- 



Fig. 155. Cepbalula stage of Echinoderms and Worms, lateral view. A, Hoio- 

 thnrian, Zf, Star-fish, C7, Z>, of Annelides. 



o, mouth ; i, stomach ; a, vent ; 0, prseoral ciliated band, in B, C, D, independent ; 

 in A 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 strobilation. This is commonly observed 



