92 ART. 4. — X. YATSU : 



of cirri appeared on the most ventral part of the arm-apparatus, 

 that is, ventral to the first pair of cirri (PL VI, Fig. 86, 1 and 

 PI. VIII., Figs. 122, 123, 128, cr n .). Since this was the last 

 pair of cirri that appeared in the larvae which I was able to 

 rear, it is quite impossible to decide whether the cirri which 

 appear afterward continue to arise in the ventro-median portion 

 of the arm-apparatus. As the result of the dorsal reflexion of 

 both extremities of the arm-apparatus the cirri situated at the 

 tip the of future arm begin to curve outward in such a way that 

 their convex sides are turned toward the median plane (PI. VI., 

 Figs. 86, 87, and PI. VIII., Fig. 128). This is the Schizolophus 

 stage of the arm-apparatus of Beecher, ('97)- 



The epistome (Armfalte) has been referred to at the precediug 

 stage as a shelf-like thickening occurring near the ventral basal 

 part of the tentacle. At the present stage (10-15 p. c.) it attains 

 an enormous size, eventually becoming the lip along the arm. 

 In the oldest larva I have been able to examine, this region had 

 assumed the shape of a depressed pentagon, overhanging in front 

 of the mouth : it here reached almost the ventral region of the 

 arm-apparatus (PL VI, Fig. 86, and PL VIII, Fig. 128, epsL). 



To turn to the internal structure of the arm-apparatus. In 

 earlier stages there appeared an arm-sinus communicating directly 

 with the body cavity. It was lacunar in character partly filled 

 with muscles and mesenchymatous cells. In the sedentary larvae 

 a strong septum now appears and shortly divides the arm-sinus 

 into two : of the cavities thus formed one elongates and becomes 

 a canal, while the other undergoes but little change. The former 

 is still widely open into the body cavity ; it will in far advanced 

 larva? be cut off from the body cavity and known as the great 



1 By mistake marked cr IJ . in Fig. 8(5. 



