BY R. J. TiLLYARt). 



103 



the two main tracheie and the mid-rib. The nuclei of the mesh- 

 work closely resemble those of the hypodei-mis in size and shape. 



The tracheal system is moderately well developed, but the 

 branch tracheie are neither so abundant nor so regularly placed 

 as in Vertical Lamellar Gills. 



These gills are frequently lost or cast oit" by the larvte, the 

 breaking-joint being very perfectly formed. Consequently larvie 

 are very commonly found with one or more regeiierated gills. In 

 these there will always be found /o^^r main longitudinal tracheie, 

 in the case of the median gill, and 

 usually three (rarely four) in the 

 laterals. The regenerated gill is 

 stouter and less expanded than the 

 normal gill, and more resembles a 

 rather flattened saccus (Plate i., fig. 

 11). Usually a transverse band of 

 dark pigment separates a thicker 

 basal portion from a thinner distal 

 portion, and suggests the origin of 

 this type from an originally two- 

 jointed form. 



The larvae of only two species are 

 be distinguished as follows: — 



Length of gills, when full-grown, U-7nnn 



Text -fig. 29.'^ 

 known, and their gills may 



tlu 



median gill ro-2iinii. 

 wide, the laterals slightly less. Colour pale brownish, semi- 

 opaque, the distal half ci'ossed by a very wide band of chocolate - 

 brown pigment; slight irregular pigmentation of the same colour 

 is present also at about two-fifths from the base (PI. i., figs. 8-9). 



A. ictero/nt/as Selys. 



Length of gills, when full-grown, :V.")-4mm., the median gill very wide, 

 r8-2mm.. the laterals nearly as wide. Colour pale l)rownish, 

 semi-opaque, with slight irregular darker pigmentation towards 

 tip (PI. i., fig.lO) J. (jristiix Selys. 



1). The Reduced (Non-funetional) Type (Text-figs. 80-3-2). 

 The Saccoid, Triquetro-quadrate, and Lamellar Types of 

 Caudal Gills are all characterised, morphologically, by the pos- 



■ Horizontal lamellar gills of A njlohstex kteromdas Selys, in natural 

 osition, dorsal view; ( x 6). 



