84 CAUDAL GILLS OF ZYGOPTERID LARVAE, 



the longitudinal nerves, and the blood-canals. In Vertical 

 Lamellar Types, the original rachis, bounded laterally by the two 

 zones of principal laminae, is not entirely confined to the thickest 

 part of the gill, under the mid-rib, but, extends slightly outwards 

 both dorsallv and ventrally. The principal longitudinal trachea' 

 then occupy the middle portion, under the mid-rib, while the 

 blood-canals and their accompanying nerves lie dorsally and 

 ventral! V within the outer portions. This arrangement is carried 

 even further in the Lesfidcf, where the rachis becomes divided, 

 by two longitudinal constrictions, into three parts, viz., a greatly 

 swollen central portion carrying mid-rib and tiacheje, and dorsal 

 and lateral extensions, only slightly swollen, each carrying a 

 blood-canal and its accompanying nerve or nerves. This is well 

 seen in Text-fig. 19. 



In Horizontal Lamellar Gills, the rachis remains undivided, 

 carrying the blood-canals and nerves ventrally, the main trachea^ 

 being arranged one on either side of the blood-canals(Text-fig.28). 



AVe shall deal with the various forms of Lamellar gills under 

 two main headings or sub-types, the first of which will again be 

 divided into a number of separate sections. These two sub-types 

 are (a) the Vertical Lamellar, and (b) the Horizontal Lamellar 

 Sub- type. 



(a) The Vertical LameUar Snh-Type (Text-6gs.l9, 22). 



This form is clearly a specialisation from an older Triquetro- 

 quadrate Type. The quadrate median gill of this latter type is 

 already practically a vertical lamella as regards its more distal 

 portion. It only lequires slight further flattening, and a further 

 differentiation between rachis and blade, to make it a true 

 lamellar type. The change which will produce a lamellar gill 

 from the triquetral lateral gill is, however, more profound, since 

 it must undergo, not only much greater flattening, but also a 

 definite change in the direction of its main axial plane. The 

 triquetral gill is suitable for resting upon a flat horizontal 

 surface, with its mid-rib outwards. In a triangular section, we 

 consider the mid-rib to occupy the vertex, so that the base 

 is the long side facing the median gill. The triangle is then 

 nearly, but not quite, isosceles. The lamellar form has been pro- 



