26 ART. 4. — N. YATSU : 



sents the epistome (Armfalte) (epst.). When the arm-apparatus 

 contracts the oesophagus is bent nearly at right angles, thus for 

 a short distance running dorso-ventrally from the mouth and 

 then taking a vertical course to the stomach. When, on the 

 contrary, the arm-apparatus stretches, the oesophagus becomes 

 almost straight. When the animal contracts the lumen of the 

 stomach is seen as a transverse slit, its anterior and posterior 

 walls being closely apposed on each other, giving the deceptive 

 appearance of the coelomic sacs being formed as enterocoelic 

 diverticula, as Kowalevsky ('73) has observed in the embryo 

 of Cistella. The mesoblast cell-masses which are placed one on 

 each side of the alimentary canal acquire a distinct lumen (coelom) 

 (cl). Concurrently with the thinning of the walls the coelomic 

 sacs come to extend all around the alimentary canal, as is seen 

 in Fig. 50. (PI. III.). As to the mantle, no special change takes 

 place except growth in size. 



At the next stage (PI. III., Fig. 52., and Fig. 53. ; both 

 ventral views) the third pair of cirri (cr nI .) arise between the 

 second pair (cr TI .) and the tentacle (tnt.) as low protuberances. 

 Later on new pairs of cirri are always added between the tentacle 

 and the cirri next to the latter. This mode of intercalation has 

 been noted by Kowalevsky ('73), Brooks ('78) and Beecher 

 ('93)- When the arm-apparatus stretches, the tentacle and the 

 first pair of cirri are directed dorsally. In the contracted state 

 the position of the cirri is constant : the first pair of cirri face 

 anteriorly ; the second pair posteriorly (PI. III., Fig. 52.). The 

 stomach (dm.) is dilated into a spacious sac, especially when the 

 animal contracts. The walls of the coelom become thinner and 

 thinner until they are hardly visible. A pair of muscles, which 

 I shall call the ventral muscles (m. vt. Fig. 53) makes their first 



