Fig. 207.— Development of Chiton Polii (after Kowalevsky). A-F, Six stages in the develop- 

 ment of the gastrula into the young Cliiton ; sections nearly median. G, frontal section through 

 stage C, oblique, from the upper part of the velum to the blastopore. H, I, K, L, transverse 

 sections of four stages of development behind the mouth. 1, Blastopore ; 2, archenteron or 

 midgut ; 3, mesoderm ; 4, ectoderm ; 5, velum or preoral ciliated ring ; 6, stoniodseum or oesophagus ; 

 7, mouth; S, radular sac; 9, body cavity; 10, pedal gland, in I asophagus ; 11, foot; 12, anus 

 with proctodeum ; 13, cerebral ganglion ; 14, pretrochal tuft ; 15, pleurovisceral cords ; 10, pedal 

 cords ; 17, mantle furrow ; IS, eye ; c, shell ; cj-c^, the seven shell plates first formed. 



