ON THE DEVELOPMENT OF LINGULA ANATINA. 93 



arm-sinus {gr. am. sn.). In a surface view the sinuses can be seen 

 to pass anteriorly along the ventro-lateral side of the oesophagus ; 

 they then diverge from each other and terminate blindly (PI. 

 VI., Fig. 86, and PI. VIIL, Fig. 128, gr. am. sn.). The 

 remaining cavity, small in comparison with the great arm-sinus 

 is entirely separated from the body cavity and gives off the 

 cirrial canal. It is referred to as the small arm-sinus (sm. am. 

 sn.). It should be mentioned that this sinus later comes into 

 communication with the body cavity by means of a pair of canals, 

 though this did not take place during the stages I was able to 

 examine. The walls of the sinus at the 10-15 p. c. stage consist 

 of stout supporting substance with a lining of thin epithelium ; 

 the latter has certainly been formed by the transformation of the 

 mesenchymatous cells found in the arm- sinus in the preceding- 

 stages. The muscles in the arm-sinus which attained their elab- 

 orate development in the free-swimming larvae, undergo regressive 

 changes in the course of the 10-1-3 p. c. stage, and become the 

 insignificant brachial (m. br.) and cirrial muscles, which subserve 

 the function of protruding the arm slightly out of the shell. 

 The retrogressive changes affecting the arm-apparatus must clearly 

 be due to functional changes at the beginning of the sedentary life. 

 The foregoing relations of the sinus and muscles will be more 

 clearly understood by reference to Figs. 124-125, Fig. 123, and 

 Fig. 138 (PL VIII.). 



The lacunar system found in the adult around the oesophagus 

 (Perioesophagealka miner) and in the epistome (Faltensinus) do 

 not as yet make their appearance. 



Nerve tissue (n.) is now seen as the thickening of the 

 epithelium which covers the ventral wall of the oesophagus ; 

 anteriorly it appears in a more lateral, and still more anteriorly 



