214 



ZOOLOGY. 



articulates, interpolated between the penultimate and ter- 

 minal segments of the body. At B, the body is many- 

 jointed, the tentacles well developed, the large temporary 

 bristles hare been discarded, and the worm can be identified 

 as a young Polydora. 



It is probable that Polydora is hatched as a trocJiosphere' 

 like that of Polyzoa, Brachiopoda and certain moUusks. 

 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, Lamellihram,cMata, Gastro- 

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

 tina, and Annelides are almost identical. Farther 

 along in their developmental history, the cepha- 

 lula of the Annelides (Fig 147, A, B, and 149),. 

 is like that of certain Echmoderms (Fig. 149),. 

 Gei)liyrea, Polyzoa, Brachiopoda, and Molhoica. It may 

 here be obsei'ved that the free-swimming larva; of these tvpes 

 of invertebrate a.nimals are the young of more or less seden- 



Fig. 

 Larva of 

 PhyUodocQ.— 

 Aftpr A. Ag- 



a&siz. 



Pi?. 149.— Cephalula stage of Echinoderms and Worms, lateral view. A., Holo- 

 thurian, B. Slar-fish, C, Z>, of Anuelides. 



0, moutli : i, stomach ; a, vent ; v, prseoral ciliated band, in .B, C, 2>, independent ; 

 in A surrounding an oral region. — From Gegenbaor. 



tarr 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-di^-ision, the 

 process being called stroiilation. This is commonly observed. 



