DEFECTIVE PLUTEUS LARVAE. 



387 



larvae listed in column (5), Table II., showed defective skeletons. 

 Two sets of these- the Arbacia pair and one of the pairs from an 

 Echinarachnius egg were larvae of the one armed type illustrated 

 in Fig. 3. The other two pairs of Echinarachnius larvae showed 

 skeletons which were clearly complementary, that is the one 

 member showed the parts which the other lacked. These two 

 pairs of larvae are shown at the age of four days in Fig. 4, A and 



A 



FIG. 4. Pluteus larvae of Echinarachnius four days old. A shows two larvae 

 developed from the two isolated blastomeres of the two cell stage of the same egg, 

 the smaller one shown from the side and the top, the larger from the top only. 

 B shows a similar pair from the top only. C shows a normal control larva of the 

 same age, aboral view. Measured and drawn free hand to the same scale from 

 living specimens. 



