BY R. J. TILLYARD. 



73 



of about equal size in all parts of the gill, this being also the 

 size of the alveolar nuclei. Dorsal and ventral blood-canals {dh, 

 vh) are well-developed, symmetrically placed in the median 

 saccus, unsymmetrically in the lateral sacci. 



Text -% 11. 

 Ideal (semi-diagrainniatic) 1\S. across theSaccoid Gill-system of Dlpldtbla, 

 to show the internal structure of the gills. Lettering as on p. 109. 

 The large interior of the saccus is chiefly occupied by the 

 immense development of the alveolar meshwork, containing 

 hundreds of spheroidal chambers, or alveoli, of somewhat vary- 

 ing size. This meshwork is supported, along two well-marked 

 zones, by the internal laminie {dl, vl), which close oflf small longi- 

 tudinal dorsal and ventral portions from the main central por- 

 tion of the gill. In these outer portions, in which we must see 

 the first beginnings of the blade-portions of the Lamellar Uill- 



