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, Bracliiopoda and certain mollusks. 

 The young T&rebrellides 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, Bracliiopoda, Lamellibranchiata, Gastro- 

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

 Fi g . 154.- tina, and Annelides are almost identical. Farther 

 ^Pbyiiodoce - al n g in their developmental history, the cepha- 

 jSfe! AlAg " lula of the Annelides (Fig. 153, A, B, and 155), 

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

 GepJtyrea, Polyzoa, Bracliiopoda, 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. Cephalula stage of Echinoderms and Worms, lateral view A Holo- 

 thurian, B, Star-fish, C, D, of Annelides. 



o. mouth ; i, stomach ; a, vent ; v, 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 



