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 identilied 

 as a young Polydora. 



'It is probable that Poly dor a 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, Bracliiopoda, Lamellibrancliiata, Gastro- 

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

 - tina, and Annelides are almost identical. Farther 

 ^ on S ^ n their developmental history, the cepha- 

 After A. Ag- l u i a o f the Aimelides (Figs. 117, A, B, and 119), 

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

 Gcphyrea, 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 sedeii- 



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

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



o, mouth ; i, stomach ; a, vent ; v. praeoral ciliated band, in , C, D, independent ; 

 In A surrounding an oral region. From Gegenbaur. 



i fcary 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 



