98 



I. IJIMA AND S. IKEDA. 



No chromatophores are found in connection with this sheet. The 

 internal surface is probably in almost direct contact with the epidermis 

 of the mantle-cavity. 



The intervening space, between the two muscular sheets above 

 mentioned, may be one millimeter or more in width and is taken up by 

 an elastic connective-tissue, which is traversed crosswise by numerous, 

 fine muscular bundles. 



The wall of the siphon is essentially of the same structure as the 

 mantle. In both sheets of its muscular tunic are found obliquely 

 running fibers in addition to the transverse and the longitudinal. 



Exceedingly remarkable seemed to us the structure of the arms. It 

 may roughly be described as a thin-walled tube. The wall, not more 

 than 0.15 mm. in thickness, is a firm muscular membrane in which the 

 compactly arranged muscle-fibers form an outer circular and an inner 

 longitudinal system or stratum, exactly comparable to those of the inner 

 muscular sheet of the mantle. Apposed to the outer surface of the 

 membrane are a number of chromatophores. The wide internal cavity 

 of the arms is crossed diametrically by a large number of fine, white and 

 glistening muscular bundles, the interspaces between which are ap- 

 parently filled up by a gelatinously metamorphosed connective-tissue, 

 quite like that of the integument except that it contains no chromato- 

 phores. By making a longitudinal incision, the muscular membranous 

 wall of the arm may be laid aside and the internal ganglionic chain 

 exposed, as if we were dissecting an earthworm or a Nematode. 



The muscular wall of the arms is of course continuous with that of 

 the suckers. The latter contain, inside the piston, a small space which 

 is shut off from the internal arm-space by a thin membranous partition 

 and which is traversed by numerous muscular bundles extending 

 between the piston and the partition just mentioned. 



Cutting open the siphon and the mantle so as to expose the visceral 

 sac, we have ascertained the following points : 



The ventral wall of the siphon and the ventral mantle, both as seen 

 on the inner surface, are continuous but still show a boundary-line 



