ss Subneural sinus . 



s St omach . 



ti First pair foot tentacles. 



t2 Buccal tentacles. 



wvr tf&ter yasc p ring. 



1, 1 1, 1 1 1, IV ,V. Arm rudiments. 



1,2,3,4,5. Radial water canals. 



PLATE I . 



Figure 1. Larva in stage "A" seen fpom the- right 



side, the right half of the ectoderm. re- 

 moved and the mesenchyme omitted. 



Figure 2. The same larva seen from the ventral side 

 as a transparent object. 



Figure 3. Median longitudinal section of a larva 

 in stag:- "A". 



Figure 4. A larva in stage- "B" viewed from the ven- 

 tral side as a transperent object. 



Figure 5. Transverse section of a lapva in stage 



"B" in a plane half way between the blas- 

 topore and the point where the hydrocoele 

 is connected with the archenteron. 



Figure 0. Larva in stage "C" seen from the ventral 

 side ,the ventral half of the ectoderm, 



