No. 480] BLOOD GILLS OF SIMULIUM 



881 



While it has not been possible to see the actual blood movement 

 in the projected gills, the abundance of blood-mass which appears 

 in the sections in all parts of their cavities clearly demonstrates its 

 presence there. 



The gills may be projected or retracted at the will of the animal. 

 When out, the middle filament projects dorso-cephalad, the side 

 filaments dorso-latero-cephalad, and all the branches are thrown 

 free into the swiftly flowing water which is the normal habitat of 

 the animal (Fig. 1, B). The projection of the gills includes not 

 only a movement of the organs themselves but also of the rectum. 

 The usual amount of movement is shown in Fig. 2, A and B. 

 Points e and e' represent the beginning of the normal intestinal 

 epithelium on the dorsal and ventral sides respectively, while d and 



represent the beginning of the body wall on the same respective 

 sides. In Fig. 2, A, the gills are retracted, while in Fig. 2, B, they 

 are projected as much as the healthy animal usually protrudes 

 them. In Fig. 2,B, e and e' have moved to the dorsal surface of 

 the body, while d and d' have retained almost the same position. 

 Sometimes, however, the living animal may project the gills to the 

 extent illustrated in Fig. 2, C, and wlien killed in hot alcohol gen- 

 erally does so. This protnusion diflVrs from that of the ventral 

 sacs in the Thysanura, wlu'ie the sac is simply turned wrong side 

 out, for these are projected without heinu- everted. 



These gills are almost if not completely witluh-awn ulien the 

 animal is moving, and project freely only when it is fastened l)y its 

 caudal sucker. They are rarely quiet when piojeetiiio- but are 

 usually moving a little way in or out. 



I have found no muscular apparatus by the action of which the 

 gills could be projected, but have been able to make them {)rotrude 

 bv pressure exerted progressively from in front backward, and, as 

 I have foiuid the hhxxl-niass in sections of protruded gills, it seems 

 onlv reas()nal)h' Id cone huh' that the gills are pushed out by an 



some part in the retraction of those organs. To aid my descrip- 



