94 CAUDAL GILLS OP ZYGOPTERID LAR\VE, 



Tliough not so distinctly two-jointed as the constricted form 

 of gill, the nodate form is easily seen to consist of two parts, 

 viz., a basal thickened portion, in which the hypoderm-nuclei 

 are ahnost all of the larger size, and a distal thinner portion, 

 with hypoderm-nuclei mostly of the smaller size. The boundary 

 between these two parts is not equally distinct across the gill 

 from one edge to the other. On that edge of the gill which faces 

 outwards in the natural position (i.e., the dorsal edge of the 

 median gill, and the ventral edges of the laterals), there is a very 

 distinct notch or depression of the edge, bordered basally by a 

 very conspicuous spine, which is the last, and the largest, of a 

 series of spines running along this edge from the very base of 

 the gill. This notch I have termed the node. 



From the node, the division between the two portions of the 

 gill is marked b}" the nodal line, which runs inwards to the 

 rachis, usually somewhat obliquely towards the base, until it 

 reaches the mid-rib, which is generally spineless. Fairly dense 

 pigmentation along the distal border of the thick basal portion 

 renders the nodal line very clearly marked in this part of the 

 gill. From the mid-rib onwards to the inner edge of the gill^ 

 the division is not so clearly marked. It can usually be traced, 

 either transversely across the gill, or slightly obliquely, for at 

 least two-thirds of the distance from mid-rib to inner edge. On 

 approaching the latter, it becomes faint, and may disappear 

 completely. This is due to the fact that the basal portion is 

 not swollen along the part bordering the inner edge, but is thin 

 like the distal portion, and merges into it without any definite 

 boundary line There are either no spines along the inner 

 margin, or only small ones running from the base to the division- 

 line. 



These gills are always somewhat narrowed at the base, whence 

 they gradually broaden out until the node is reached. Beyond 

 the node, after the slight constriction, the gill-blade either again 

 broadens out, ending in a very broadly rounded tip, or the two 

 edges remain parallel, with the tip well rounded off, but not 

 broadened. The rachis and mid-ribs always lie much closer 

 to the outer than to the inner edge of the gill, and are only 

 poorly developed beyond the level of the node, 



