172 



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, BracMopoda and certain mollusks. 

 The young Terebrellides fitroemii, 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. 215), Nemer- 

 . tina, and Annelides are almost identical. Farther 

 doce - a l n g ^ n their developmental history, the cepha- 

 After A. Ag- hila of the Annelides (Figs. 117, A, B, and 119), 

 is like that of certain Echiuoderms (Fig. 119), 

 Gepliyrea, Polyzoa, Brachiopoda, and Mollusca. It may 

 here be observed that the free-swimming larva? of these types 

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



Fig. 119. Cephalula stage of Echinoderms and Worms, lateral view. A. Holo- 

 thunan, B, Star-fish, O, D, of Anuelides. 



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 



