ON THE DEVELOPMENT OF LINGULA ANATINA. 89 



128, nph.). The opening of the nephridium to the exterior is 

 shown in Fig. 135 (PI. VIII.). 



Otocysts. — The otocysts become more conspicuous than before. 

 In /o/o-preparations we see these structures as distinctly closed 

 sacs whose walls are made up of a thin epithelium (PI. VI., Fig. 

 87, ot.). The otocysts persist throughout life as will be seen 

 in my paper on the adult histology (loc. cit.). This observation 

 does not agree with that of Fritz Müller in the larvœ of 

 Discinisia ('61). But this discordance must be due to the generic 

 difference. 



Nervous system. — As to the ganglia there occurs no per- 

 ceptible change except the increase in size. That part of the 

 ventral body wall which is situated between the points of origin of 

 the occlusores anteriores and gives the attachments to the obliqui 

 interni, forms a recess directed posteriorly. On the proximal 

 wall of this recess there are seen a pair of longitudinal thickenings 

 which, without doubt, constitute the Nervus peduncular is. The 

 above recess is in all probability transformed into the closed sac 

 covered with a thick layer of supporting substance which is found 

 at the insertion of the obliqui interni in the adult. 



Muscles. — During the 10-15 p. c. stage the shell muscles 

 tend more and more to assume the adult form and undergo no 

 important change except the disappearance of the M. ventrales, 

 the 31. dorsales having vanished prior to the 7 p. c. stage. The 

 M. ventrales attained their maximum growth during the preced- 

 ing stage and now, concurrently with the change in mode of life, 

 become very inconspicuous and eventually disappear. In one 



