92 CAUDAL GtLLS 6V ZYGOPETRlt) LaRV^, 



a. The Constricted Vertical Lamella. In tliis, the two original 

 joints are present, separated by a very distinct suture or con- 

 striction. 



/i. The Nodate Vertical Lainella. The two original joints have 

 fused into one, but their point of union is marked by a node of 

 the type described below (p. 94), which is formed by the reduc- 

 tion of the original suture or constriction. 



y. The Subuodate Vertical Lamella. The node is only in- 

 dicated by a change in the thickness of the gill, together with 

 the stopping short of the cuticular spines at the point where the 

 node once was. 



8. The Denodate Vertical Lamella. All traces of the node are 

 lost, except, perhaps, for a slight change in the thickness of the 



gill. 



a. The Constricted Vertical Lamella (Text-lig.23; Plate i., iig.7 ). 



This form of gill only occurs, to my knowledge, in the genus 



Isosticta of the Protonenrvncp.. It is of very great interest, not 



only because it is a definitely 

 two-jointed lamella, but also 

 because it stands exactly half- 

 way between the constricted 

 or two-jointed saccus of Neo- 

 sticta and the Nodate Verti- 

 Text-fifr.28. ^ cal J^amella found in the tribe 



Coiistiietcd gills of L'<osficta .^imp/t.c Psenda<jrionini. This change 

 Maithi, in luituial position, lateral jg correlated with a change in 

 ^"^^^^' ^ '' ^^' the habits of the laiv*. Neo- 



sticta is, as we have seen, a rock-dweller in running water. 

 Isosticta dwells in trash, twigs, or matted roots, on the bottoms 

 of slowly-moving streams. J n the position of rest, it holds its 

 irills well awav from tlie resting-surface. Occasionally it climbs 

 up into clumps of water-weed, in which case the larva becomes 

 brown instead of almost jet-black (its natural colour). In 

 slenderness of form, it is a typical Aijrionid larva. 



Our original simile of the "candle and Hame," used to describe 

 the appearance of the gills of Neosticta, would hold fairly well 

 for Isosticta, if we add that, in this case, the Hame is as big as 



