70 



CAUDAL GILLS OF ZYGOPTERIB LARV.E, 



ventral one passes slantingly downwards to enter 



the base of 



the lateral gill of its own side. On entering the gill, tliis branch 

 again divides into two, in all except the Lestid sub-type of gill* 

 The more dorsal branch runs straight back through the tenth 

 segment to enter the base of the median gill, alongside its fellow 

 from the other dorsal trunk. Thus the median gill receives two 

 main tracheal stems at its base. In all Lamellar Types of gill, 

 these two trunks run alongside one another through the gill, 

 giving off numerous branches. In the Saccoid and Triquetro- 

 quadrate Types, however, each divides, on entering the gill 

 proper, into two large tracheae, of which one is placed dorsally 

 above the other. Thus, in these types, the median gill has four 

 parallel main tracheae running through it. This arrangement is 

 shown diagrammatically in Text-fig. 8. 



The courses of the main tra- 

 cheae, their manner and frequency 

 of branching, and the form of their 

 smaller branches or "twigs," vary 

 so much that it seems advisable to 

 leave the description of these parts 

 under the sections dealing with 

 the separate types of gills. 



The structure of the tracheae in 

 the caudal gills does not differ in 

 any respect from that of tracheae 

 in any other part of the larval 

 body. Being derived from the 

 dorsal trunks, they are, like these 

 latter, always deeply pigmented, 

 the pigment-granules lying in the 

 ectotrachea. This pigmentation 

 is found throughout the tracheal 



Text-tig. 9.^ 



system, from the main stems down to the final branches from 



* T.S., through a main trachea in gill of Galopteryx sp. ( x 370), to show 

 reduced portion of hfemoccele {he), [containing blood-plasma {Up) and 

 aiiifvbocytes (ac)], in which the trachea lies, al, alveolus; ect, ectotrachea. 

 end, endotrachea; il, internal lamina; nui, nucleus of tct; nti^, nucleus 

 of end. 



