74 ART. 4. — N. YATSU : 



body wall, and running across the body cavity dorso-ventrally. 

 They are probably formed from the dorsal body wall. In a living 

 larva of the 6 p. c. stage I observed that the occlusores anteriores 

 are formed at first as a group of short rods attached to the dorsal 

 body wall. 



The ventrales have already been met with from the 3 p. c. 

 stage (PI. III., Fig. 53) onward. These muscles arise on the 

 ventral shell, a little mediad of, and posterior to, the origin of 

 the occlusores anteriores (PI. VI., Figs. 81, 82, Wood-cut A, m. 

 vt.). Running forward they are inserted on the antero-ventral 

 side of the oesophagus, where they unite with the transverse 

 arm-muscle (m. br. tr.). It should be noted that the insertions 

 of the muscles have greatly shifted ventrally, travelling half 

 around the oesophagus. 



The dorsales (PI. VI., Fig. 81, Wood-cut A, m. dr.) are the 

 only muscles which have their origin on the dorsal shell. They 

 are originate at the same level as the otocysts, but nearer the 

 median plane. They then proceed forward diverging, passing just 

 mediad of the occlusores anteriores until they join the transverse 

 arm-muscles. The above two pairs of muscles, ventrales and 

 dorsales, subserve the retraction of the arm-apparatus as a whole. 

 These pairs are not present in the adult and are to be regarded 

 as larval muscles. 



The obliqui interni (PL VI., Figs. 81, 82, Wood-cut A, obi. 

 int.) show but little difference from those of the stage next to 

 be described. Their origins lie on the ventral shell very near 

 the median plane at the same level as the occlusores anteriores. 

 They then run dorsad and posteriorly, diverging as they go back- 

 wards, and finally are inserted on the dorsal shell. When the 

 body wall contracts, these muscles often appear as if their posterior 



