84 AKT. 4. — N. YATSU : 



of supporting substance, its thickness varying greatly in different 

 parts. The great arm-sinus of the adult has not as yet appeared 

 even in the larva of the 9 p. c. stage. The arm-sinus of this 

 stage is, however, certainly not the small arm-sinus as stated by 

 Blochmann ('98) in the larvae of Disc'misca (p. 422) : it is 

 potentially rather the small arm-sinus plus the great arm-sinus, 

 because the latter does not appear de novo, but is later divided 

 off from the early sinus by the growth of a septum. 



In the arm-apparatus there are two kinds of muscles : the 

 one being the longitudinal arm-muscles, and the other the trans- 

 verse arm-muscles. The former (PI. VIL, Fig. 95, m. br. Ig.) is 

 composed of the cirrial muscles and the tentacular muscles. The 

 cirrial muscles of the dorsal group of cirri joined by the tenta- 

 cular muscles run ventrally and posteriorly like the ribs of a fan. 

 The cirrial muscles of the ventral group of cirri, on the other 

 hand, are each divided into two slips, one of which comes to 

 unite with a similar slip from the cirrus adjacent on one side, 

 and the other with that on the other side. The united slips of 

 all the cirrial muscles of the ventral group of the cirri come 

 together with the muscles from the dorsal group of cirri, and 

 run posteriorly along the ventral wall of the anterior prolongation 

 of the body cavity (f f), forming the anterior part of the m. 

 ventralis (PI. VIL, Fig. 93, in. vL). The transverse arm-muscles 

 lie transversely along the ventral margin of the arm-apparatus, 

 being attached at their extremities to the cirrial muscles of the 

 lateral and posteriormost pair of cirri (PI. VI. , Fig. 81, in. 

 br. ir.). 



