80 AKT. 4. — N. YATSU : 



circular layer of muscle continuous with the muscle r [occ. post.']" 

 (p. 65-66). One might naturally come to this opinion if surface 

 preparations alone were studied. Moreover Brooks ascribed the 

 wrinkled appearance of the peduncle to " the contraction of 

 the circular muscles of its wall at somewhat regular intervals " 

 (p. 66). On examining sections we can soon find that such 

 circular muscles are not present at all. Within the inner layer 

 of the peduncle there is a narrow canal which can be shown to 

 be a prolongation of the body cavity. It communicates with the 

 latter through an elliptical opening compressed dorso-ventrally, a 

 little to the right of the occlusor posterior. 



It will be seen from the above description that the peduncle 

 of Lingula is quite different from that of the Testicardines in 

 its structure as well as in its mode of development. 



n. Arm-apparatus. 



I shall employ the term, arm-apparatus (Blochmann), to 

 indicate collectively the lophophore and the tentacles of Brooks. 

 The entire apparatus is of a very light yellow or brownish color 

 (PI. V., Figs. 77-79, and PI. VI., Figs. 83, 84). In younger 

 larva? the tips of the tentacle and the cirri are colored yellowish- 

 brown. The arrangement of the cirri when the larvae are at rest 

 is to a certain extent definite, as Brooks has stated : the ventral 

 three or four pairs of cirri form the ventral group, while the rest 

 constitute the dorsal. When the arm-apparatus is fully extended 

 it takes the form of a funnel compressed dorso-ventrally, as 

 already described in connection with the habits of the free- 

 swimming larvae (PI. VI., Fig. 88, a, b). 



I shall now refer first to the tentacle and next to the cirri. 



