Plate XXIV. 



{Top) Left, the nauplius stage, and riglit, the cypris stage of the acorn barnacle, Balaiius balatioides. 



{Bottom) Left, the zoea stage, and right, the megalopa stage of crabs. 



Photographs: 3 by D. P. Wilson, niegalopae by James Fraser 



Other and arc called 'plutei'. These, too, gradually metamorphose (Plate 

 XXVII). Fertilized sea urchin eggs, in particular, are easy to obtain by mixing 

 the eggs and sperm in sea water and have been much used for physiological 

 experiment. 



To end this chapter mention should be made of the larvae of the bottom- 

 living members of the Chordata, excluding fish. These are the worm-like 

 Hemichordata such as Balatw(jlossiis which has a 'tornaria' larva (Fig. 26; 8: 

 Plate XXIII) and the attached Tunicata, the ascidians or sea squirts which have 

 a 'tadpole' stage (Plate XX). These ascidian tadpoles (and those of the 



G 91 



