The Morphology of the Tadpole of Xenopus laevis. 
243 
there are no other gills." This is incorrect, as will be shown later under 
the heading " Operculum," but the gills cannot be respiratory, since they 
receive no special blood supply (see Gl and G2 of Fig. 5). 
The lungs are developed and supplied with pulmonary arteries and 
veins at a very early stage, but they remain sacs with such delicate walls 
that it is not probable that they are the only organs active in respiration. 
Also, the tadpoles do come to the surface to swallow air occasionally, but 
they can remain under water for such long periods that the air cannot be 
their only source of oxygen. 
The statement of Marshal and Bles that the blood from the filtering 
apparatus is drained into somatic veins, holds good also for Xenopus ; for 
/V. 
Fig. 1. — Et = Ethmoid cartilage ; N = nasal sac ; Mj = muscle band attached 
anteriorly to dorsal edge of Meckel's cartilage, posteriorly to the hinder dorsal end of 
the pterygoid cart. ; muscle band from Mj to tentacle ; MC Meckel's cart. ; TC cart, 
from palatine to tentacle; M3 muscle from outer edge of hyoid to Meckel's cart. ; S. 
mandibulo-hyoid slit ; Gi thymus ; Hy hyoid bar ; M4 " respiratory " muscle. 
the lingual, external jugular and precaval veins (see Fig. 8) receive the 
blood from the three pharyngeal grooves, but I do not see why the 
respiratory function of these grooves should on this account be denied, 
especially when it is seen that they are so richly supplied with blood 
(Al-6 Fig. 7). 
Gill-slits and Associated Glands. — Beddard noticed the deep pouch 
situated just behind Meckel's cartilage, and the lateral prolongations of this 
pouch represented, he thought, the mandibulo-hyoid cleft. Fig. 1 shows 
19 
