W. H. FURLONGE ON THE PULEX IRRITANS. 201 



contains two spiracles, one on each side, — the whole series constitut- 

 ing two rows of these organs, ranged horizontally, about half way 

 between the medial line and the top of the abdomen, on either side 

 of the animal. The first or anterior pair of abdominal spiracles, 

 however, is situated considerably above the general line, and much 

 closer to the top of the abdomen. The superior terminal plate 

 contains two very curious spiracles, presently to be described. In- 

 clusive of the pair of spiracles, previously mentioned as situated 

 in the mezo-thorax, therefore, the external respiratory organs of 

 the flea consist of 18 spiracles, of which those contained in the 

 seven anterior plates of the abdomen are almost identical in size 

 and structure. They consist of nearly circular orifices in 'the 

 chitinous plates, from the inner margins of which about eight or 

 ten short, stiff hairs radiate, horizontally, towards the centre, thus 

 forming a protective fringe over the openings. The orifices open 

 into funnel-shaped cavities beneath, terminating in short tubes, 

 which are connected to the main trachea in a peculiar manner, 

 subsequently to be described. A remarkable development of the 

 external respiratory organs, is found in the two spiracles just men- 

 tioned, as situated in the eighth or terminal superior plate of the 

 abdomen. Around the margin of the excavation in this plate, 

 (within which the pygidium is set,) and curving downwards on each 

 side, is situated a long narrow groove or channel, (apparently formed 

 by a duplication of the edge of the plate itself), which is thickly 

 set with a fringe of short, stiff hairs. Near the superior termina- 

 tion of the channel, on either side, it is enlarged and deepened, so 

 as to form a pair of trumpet-shaped funnels, the larger orifices of 

 which are directed backwards — the smaller orifices being extended 

 into short tubes, similar to those of the lateral spiracles, which are 

 united to the main trachea, of which, in fact, they appear to con- 

 stitute the terminal external orifices. 



The Pygidium. — I now come to the description of the pygidium. 

 (See Fig. 1.) This curious organ consists of a thick, soft, fleshy 

 mass, of a light-brown colour, in shape something like a saddle, — 

 the thickened rounded flaps of which descend on either side, the 

 upper portion forming a sub-angular ridge parallel with the line of 

 the abdomen. As already stated, it is set within the oval excavation 

 in the superior terminal plate. The organ rises from, or is attached 

 to, the surface of the epidermis, or the external fleshy portion of the 

 animal's body, and is capable of free movement, sometimes being 



