ON THE DEVELOPMENT OF LINGULA ANATINA. 53 



region cell boundaries become invisible. It is certain that diges- 

 tion is intracellular. Diatoms and other unicellular algœ, as a 

 rule, are found imbedded in the liver cells (PI. VII., Fig. 101). 

 It may be remarked that this mode of digestion goes on 

 throughout later life : in the adult diatom tests are found even 

 at the blind end of the finely branched liver lobules. While 

 it is clearly the fact that intracellular digestion occurs, I have 

 no reason to deny the presence of extracellular digestion. Yet 

 I believe that it is more profitable and more economical to 

 digest food particles within the cell than outside it, as the 

 stomacal cavity is widely open to the exterior and admits freely 

 the water from outside. Cilia are not found on the liver cells. 

 Caldwell ('82) states that in Actinotrocha intracellular diges- 

 tion prevails in ciliated cells, but this difference may readily be 

 ascribed to the difference in the types. Besides food particles a 

 great number of highly refracting globules of various sizes are 

 found in the liver cells (PL VII., Fig. 102). That these globules 

 are oil drops, as Brooks states, is proved by their solubility in 

 alcohol and by their being blackened by osmic acid. These 

 globules, therefore, are indicated by the presence of vacuoles in 

 preparations treated in the usual way. 



As above stated, the liver at the 5-6 p. c. stage comes to be 

 constricted so as to form three lobes. The two diverticula on 

 the dorsal side are to be known respectively as the anterior dorsal 

 lobe and the posterior dorsal lobe ; the one on the ventral side is 

 the ventral lobe (Wood-cut p. öl.). 



The anterior dorsal lobe is the last of the three lobes to 

 develop. At the 6 p. c. stage the Anlagen of the lobe appear as 

 a pair of out-pocketings of the liver, one on each side of the 

 oesophagus (PI. VI., Figs. 81, 82, an. dr. lv.). They afterward 



