234 



ZOOLOGY. 



articulates, interpolated between the j)enultimate 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 j)robable that Polydora is hatched as a trocJiosphere 

 like that of Polyzoa, Brachiopoda and certain mollusks. 

 The young Tereirellides Stroemii, and of Linnhriconereis, 

 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, LameUibrancMata, Oastro- 

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

 tina, and Annelides are almost identical. Farther 

 along in their developmental history, the cej)ha- 

 lula of the Annelides (Fig. 153, A, B, and 155), 

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

 Geypihyrea, Polyzoa, Brachiopoda, and Mollusca. It may 

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

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



Larva of 

 Phyllo(k>ce.— 

 After A. Ag- 



asBiz. 



Fig. 155.— Cephalula stage of Echinoderms ana Worms, lateral view. A, Holo- 

 thunan, B, Star-fish, C\ Z>, of Annelides. 



0, mouth ; i. stomach ; «, vent ; v^ prjeoral 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 

 dose in-and-in breeding is prevented. 



Certain Annelides sometimes multiply by self -division, the 

 process being called strolilation. This is commonly observed 



