112 



INVERTEBRATA 



CHAP. 



We have now traced the divisions of the egg to the formation of 

 an oval embryo, at the lower pole of which is an ectodernial stomo- 

 daeum opening above into an irregular endodermic sac lined by small 

 cells, all trace of the yolky cells of a former stage having disappeared 

 whilst the upper pole of the egg is occupied by a plate of extremely 

 small cells, beneath which are ganglion cells. 



8 



FIG. 87. A, dorsal, and B, ventral views of the free-swimming larva of Yungia. (After Lang. ) 

 1-8, ciliated lobes ; o, mouth. 



Ciliated lobes now appear just below the equator ; there are eight 

 of these, and from their position they must be formed by the de- 

 scendants of laP-ld 2 . The larva, which has rotated in the capsule 

 by the action of these cilia, now escapes and leads a free-swimming 

 life. It gradually undergoes changes which have not been followed 

 by Surface in detail, but which Lang has described in Yungia. 



METAMORPHOSIS OF THE LARVA OF YUNGIA 



Already at the time of its escape from the egg capsule the larva 

 has lost its primordial radial symmetry. The mouth, which originally 



occupied the lower pole of the 

 embryo, has become shifted on 

 to one side, which afterwards 

 turns out to be the ventral side, 

 owing to inequality in the rate 

 of growth of the two sides of 

 the embryo (Fig. 86, C). Of the 

 eight ciliated lobes which have 

 been formed, one overhangs the 

 mouth, one is mid-dorsal, and six 

 are lateral, forming three pairs, 

 the corresponding members of 

 which are situated to the right 

 and left of the middle line (Figs. 

 87, 88, 89). The band of cilia 

 which fringes these lobes is continuous from one lobe to another, 

 so that it forms a continuous girdle surrounding the larva. As the 

 larva grows older it becomes longer and natter; eye-spots (oc. Fig. 



FIG. 88. Lateral view of the free-swimming 

 larva of Yungia. (After Lang. ) 



