J 98 W. H. FURLONGE ON THE PULEX IRRITANS. 



packed, internally, with a dense mass of large, striated, muscular 

 fibres, constituting the bi-penniform muscles attached to the ten- 

 dons, which move the successive divisions of the limbs. These 

 tendons extend to the extremities of each leg, and are attached to 

 the long curved claws before mentioned, which are powerfully con- 

 tracted by them. Along the line of the tendon runs a well defined 

 tracheal tube, from which minute fibres branch in every direction 

 and permeate the muscular substance. 



I have here to describe a most remarkable pair of contractile 

 sacs, which are to be found in the tibia and upper tarsal joint of 

 the third pair of legs, and, I believe, also exist in the correspond- 

 ing joints of the two anterior pairs in a less marked form ; my rea- 

 son for this opinion being that, in the cat flea, I have distinctly 

 seen these sacs developed to an almost equal degree in all three 

 pairs of legs, though they are not to be made out with certainty, in 

 the two first pairs of legs of the pulex irritans. 



The larger and more important of these curious organs is 

 situated in the upper tarsal joint, through which it extends for the 

 greater part of its length. It consists of an elongated, ovate, 

 striated sac, through the axis of, which runs the main tracheal tube 

 supplying the limb. I have not hitherto been able to make out 

 that there is any direct connection between the sac and the tracheal 

 tube. The smaller organ is situated in the tibia, and consists 

 of a long, flattened, membranous bag or sac, also surrounding 

 the main tracheal tube, but it is not striated, nor, in my opinion, 

 contractile. 



Having watched the movements of these organs, in the living 

 animal, by the hour together, I have no hesitation in describing 

 them, strange as they must appear. The action of the contractile 

 sac of the upper tarsal joint, is first, by slow distention, to become 

 filled with air, the membranous sac of the tibia simultaneously col- 

 lapsing. When fully distended, the tarsal sac suddenly contracts 

 to about one fourth its previous diameter, when, at the same mo- 

 ment, the membranous sac of the tibia becomes fully inflated. 

 This rythmical, alternate, movement sometimes proceeds, regularly, 

 at the rate of two or three pulsations in the minute, but this is not 

 always the case, as I have frequently found that it is suspended 

 for longer or shorter periods, and in many specimens it is altogether 

 wanting. 



Believing that these remarkable organs have not hitherto been 



