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, Brachiopoda and certain mollusks. 
The young Yeredrellides Stroemti, and of Lumbriconereis, 
are at first trochospheres, 7. ¢., the free-swimming 
am germ is spherical, with a zone of cilia, two eye- 
cst spots, and no bristles. Thus the earliest stages of 
a, Polyzoa, Brachiopoda, Lamellibranchiata, Gastro- 
*f poda, and even of a. Cephalopod (Fig. 215), Nemer- 
Fie, 11e.— tina, and Annelides are almost identical. Farther 
Lar ace along in their developmental history, the cepha- 
eer A.Ag- lula of the Annelides (Figs. 117, A, B, and 119), 
is like that of certain Echinoderms (Fig. 119), 
Gephyrea, Polyzua, Brachiopoda, and Mollusca. It may 
here be observed that the free-swimming larve 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- 
hurian, B. Star-fish, C, D, of Annelides. : 
0, mouth; é, stomach ; a, vent ; ¥, preoral 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 
