LARVAE AND METAMORPHOSES 23 



The flat side is now the ventral surface, with the mouth not quite 

 at the front end, the region in front of it being called the pre-oral 

 lobe, the anus being nearly at the hind end, and the curved surface 

 being the back, or dorsal surface of the larva. The cilia, which at 

 first covered the whole of the outer surface nearly equally, become 

 longer and stronger on a curved band surrounding the mouth, and 

 nearly, or completely, disappear elsewhere. As there is a front end 

 and a posterior end, a dorsal and a ventral surface, and a right and 

 left side, the larva shows what is called bilateral symmetry, and is 



FIG. ii. Larvae of a Starfish : to the left a Dipleurula, to the right a 

 Bipinnaria, from the ventral surface. (After MORTENSEN ; much 

 magnified.) 



called a dipleurula. These larvae move about in the water rather 

 actively, propelled by the cilia, feed greedily on floating micro- 

 scopic plants and animals, and as they grow, change into fantastic 

 shapes, different in the different groups of echinoderms, and so 

 unlike the adult form that many of them were described and named 

 before it was known what they were (Fig. n). After a few weeks 

 they become sluggish, cease feeding, anchor themselves to rocks or 

 weed, and pass into the adult by a sudden metamorphosis, the 

 details of which differ in different species. It is always, however, 

 only a part of the larva that grows into the adult, the remaining 

 portion shrivelling up, or being cast off. In the starfish, for instance, 

 the attachment takes place by the end of the pre-oral lobe, which 

 forms a sort of stem from which the body of the larva projects, and 

 the young starfish appears on the left side of the larva, the organs 

 of that side forming the greater part of its structure, so that the 



