ON THE DEVELOPMENT OF LINGULA ANATINA. 91 



Schmiedeberg ('82) proved it to be chitin. Ekmann also says 

 ('96) : " Ihr [Lingula-Stiel] Chitin wird von Kalilauge nur wenig 

 beeinflusst, löst sich aber fast vollständig in siedender Salzsäure," 

 (p. 233). The gelatinous substance secreted from the peduncle 

 is, therefore, either another kind of chitin or some sort of al- 

 buminoid mateiial. 



In most parts of the peduncle a distinct canal is found, 

 lined with a cylindrical epithelium, while in some other .portions 

 the peritoneal epithelium is somewhat mesenchymatous, the lumen 

 being almost obliterated (PI. VIII., Figs. 136, a, b, 137). 



Arm-apparatus. — Striking changes take place as well in the 

 external appearance as in the internal structure of the arm- 

 apparatus. When free-swimming life comes to an end the arm- 

 apparatus loses its locomotive function and acquires a new one, 

 for it then becomes an organ for causing the circulation of water 

 in the mantle cavity. Correlated with this change the neck 

 region and the longitudinal axis of the arm-apparatus become 

 relatively much shortened, and the two lateral extremities of the 

 latter are bent dorsal ly, foreshadowing the arm of the adult. 



The tentacle becomes greatly reduced as in the case of Dis- 

 cinisca (Fritz Müller) and of Glottklia (Brooks). It remains 

 for a time as a prominence on the top of the epistome (PI. VI., 

 Fig. 86), but it gradually diminishes in size until at the 15 p. c. 

 stage no trace of it can be seen even in longitudinal sections 

 (PI. VIII., Fig. 122). The tentacle, therefore, is only a larval 

 organ. 



As to some changes affecting the cirri. In the preceding 

 stages the only portion in which new cirri arose was at the base 

 of the tentacle. At the 10-15 p. c. stage, however, a new pair 



