156 



MR. R. J. TILLYARD ON THE RECTAL BREATHING-APPARATUS 



we get a total of 12 X IG x 8 x 50 or 70,800 complete capillary loops in the 

 gill-basket ! This enormous number, together with the great increase in 

 pigmentation, represents a very distinct evolutionary advance over the 

 Implicate Type. 



Origin of the Foliate Type. — This type is easily derivable from a simple 

 form of the Implicate Type. If each gill in the latter type were to become 

 constricted off below its middle, it would quickly take the form of a separate 

 leaf^ supported on a stalk in which the efferent trachea ran. The " frills " 



-cm 



Text-fig. 8. — jEschna brevistyla, Rainb. 

 Transverse section through extreme posterior end of gill-basket, showing the reduced and 

 highly-pigmented gills lying between the so-called " rectal glands." Section cut from 

 a larva which had just completed its ecdysis into last larval instar. Semi-diagram- 

 matic. ( X 72.) cm, circular muscle ; D, mid-dorsal position ; dg, duplex gill-system ; 

 pg, pigmented rectal epithelium ; R, rectum ; rg, raised epithelium of " rectal gland " ; 

 St, supporting-tissue ; V, mid-ventral position. 



on the edge or " lip " of the groove would then correspond to the " crinkles " 

 on the leaf. Finally, if each separated groove became- elongated trans- 

 versely to the longitudinal axis of the gill-basket, and came to lie more 

 transversely to that axis, we should then obtain a strictly foliate type of 

 gill-structure, only differing by the absence of fat-globules and dark pigmen- 

 tation from the foliate mil of JEsclina, 



