THE PROBLEMS OF TRANSFORMATION 245 



the great Vertebrate assemblage the highly-organized, typically 

 terrestrial or aerial classes of Mammals and Birds have no 

 outward metamorphosis, while the partly aquatic and lower- 

 grade class of Amphibia usually pass through a marked 

 transformation. All vertebrates, like animals generally, start 

 their development from a fertilized egg ; the process of growth 

 and change in a bird or a mammal is embryonic, going on 

 within the egg-shell or the mother's womb, while in the life- 

 history of the frog the young creature begins its free life as a 

 comparatively undeveloped larva, which to some extent may 

 be regarded as a precociously-hatched embryo. 



ar 



an 



FIG. 119. 



a. Larva of Starfish in gastrula stage (sectional view) (ec, ectoderm with cilia ; en, 

 endoderm lining digestive cavity (ar) ; bl, blastopore) ; b, trochophore larva of mollusc 

 (c, ring of cilia ; m, mouth ; ar, digestive tract ; an, anus). A about 100. 



If we turn to less highly-organized groups of invertebrates, 

 which inhabit the sea, such as the Coelenterata (zoophytes and 

 sea anemones) or the Echinodermata (starfishes and sea- 

 urchins), we find that the developing young usually begins its 

 free life in an extremely early embryonic stage a minute 

 hollow sac with two cell-layers known as a planula or a 

 gastrula (Fig. 119 a) a stage passed by the frog's tadpole long 

 before the time of hatching. Among more advanced groups, 

 such as the Annelida (segmented worms) and the Mollusca 

 (" shellfish " oysters, mussels, whelks, etc.), the members of 

 most of the marine families begin free life as a trochophore 

 larva (Fig. 119 b), in which there is a definite mouth and gullet 

 leading into the digestive cavity, and a special locomotor organ 

 in the form of a circlet of cilia around the broad head-region. 



