^ 148 



Eine ziemlich ausfülirliche , bis zum Jahre 1834 sich erstreckende, Geschichte der Krätzmilbe, und eine Beschreibung 

 der Krätzmilbe finden wir in Chapter XXIV, welches pag. 707 beginnt. Die Krätzmilbe, die er selbst einer näheren Unter- 

 suchung unterworfen hat, beschreibt er pag. 720, hier heisst es: „The existence of the acarus scabiei is without question; 

 I have extracted as many as twenty from their retreat at a Single sitting. 1 have placed tbem on a slide of glass , and seen 

 them run; and after the business of the day has been over, I have examined them with the microscope, and found them still 

 active, living for several hours after my examination. I have already stated that I regard them as the unique cause of Scabies, 

 and as a necessary feature in the diagnosis of that disease. 



„When examined with the nalced eye the acarus looks white and shining, globular in form, and very aptly resembling 

 the little bladder of water of Bonomo. There is no difflculty in extracting it from the skin ; the cuniculus is seen without 

 difßculty; the end of the cuniculus is perceived to be a little raised, white a greyish speck is seen beneath it. As soon as 

 Ihis little eminence of epidermis is lifted , if the end of the needle or pin with which the Operation is performed be examined, 

 the minute, white and shining giobe will probably be observed attached to the Instrument. If there be no such object, the 

 point of the needle placed again beneath the raised capsule of epidermis will pretty certainly draw it forth. This facility of 

 extracting the little creature is due to its great power of clinglng to any object with which it comes in contact. 



„When the acarus is seen running upon the surface of a plate of glass, it may be perceived that its anterior margin 

 presents a dusky tint of colour, and the examination of this part of the creature with the microscope brings into view a head 

 not unlike that of a tortoise, and a pair of large and strong legs on each side of the head. These organs are encased in a 

 moderately thick layer of chytine and have consequently the reddish-brown tint of the cases of certain insects, or of the bright 

 part of a thin layer of tortoise -shell. Proceeding with cur examination, we perceive the general outline of the animal to be 

 subrotund, the antero- posterior predominating very little over the transverse diameter; the anterior part of the creature beeing 

 broad, and the posterior some what narrower and semicircular. The ventral surface of the acarus is flat, and occupied by the 

 head and eight legs; the dorsal surface is arched, uneven, and covered by numerous spines; and projecting backwards from 

 the posterior segment of the animal are twelve hair like fllaments, some long and others short. 



,,With the View of determining the size of the acarus, I measured ten specimens, and found them vary between %47 

 and % of an inch in length, and between %47 and %^ in breadth. 



,, Examined with a quarter or eighth of an inch object- glass, or with Po^^^//'^ half- inch, the case of the body of the 

 acarus is seen to be. composed of narrow plates, variously disposed with regard to the axis of the animal, but chiefly trans- 

 versely, and resembling a coat of plait armour. The connecting membrane of these plaits permits of a certain degree of mo- 

 vement between them. The dorsum of the creature is convex, but uneven, and exhibits upon its borders a tendency to 

 division into a thoracic and an abdominal segment, the former being some what broader than the latter. Anleriorly the dorsal 

 case terminates in a sharp border, which is scalloped, and forms a justing roof of protection to the head, and each of the four 

 anterior legs. Posteriorly, the case is some what deeply cleft, forming a groove, which corresponds on the ventral surface 

 with the sexual and anal aperture. 



„The dorsal surface of the creature is covered with tubercles, spines, and hair bearing tubercles, regularly and very 

 remarkably disposed. The venter of the acarus is flat, and the abdominal portion slightly convex. The posterior part of 

 the laiter is grooved upon the middle line, and furnished with an anal and sexual aperture, of considerable size. 



,,The head is an oblong cylinder, more or less obtusely pointed in front, flattened beneath, enlarging slightly late- 

 rally towards the body of the creature , and emplanted by its posterior end into the angular interval left by the Separation of 

 the anterior pair of legs. The lateral enlargement towards the root of the head is the most suitable place for eyes 5 but I have 

 not as yet been able to detect those organs. The head is surmounted by two rows of stiff hairs. The mouth is an oblong 

 aperture situated upon the undersurface of the head , and becoming broad towards the root of the latter. Its borders are 

 furnished with a thick fringe of mandibles, and the interior supplied at each side with a number of strong maxillae. The head 

 is capable of elongation or retraction beneath the dorsal plate or carapax. 



,,The legs are eight in number, four being anterior, and four posterior; the anterior legs are large and powerful, 

 the posterior small. The anterior pair of legs are so large, so closely placed to the head, and directed so immediately forwards, 

 as to deserve the appellation of arms. The next pair follovv immediately on the proceeding, but are directed outwards. The 

 legs are conical in form, tapering, when extended, to an obtuse point, and composed ofa hip-piece and three circular Segments. 

 The hip-pieces of the two anterior legs join at an obtuse angle, and form the limit of the root of the head. The point of 

 meating of these hip-pieces is the commencement ofa sternal crest, which runs backwards on the plastrum for a short distance, 

 and terminates by a rounded extremity. A similar crest is formed at each side by the junction of the hip-pieces of the anterior 

 and lateral leg, the crest being directed backwards and inwards towards the termination of the sternal crest. Each of the 

 annular segments of the anterior legs is furnished with three or four bristly hairs , which stand out at right angles from the 

 segment. Moreover, the extremity of each anterior leg is provided with a tubulär cylinder (tarsus) as long as the entire leg, 

 and terminated at its extremity by a foot divided on its solo into flve lobes. 



,,The head and four anterior legs are covered with a strong case of chytine, which presents the ordinary colour of 

 insect cases, namely, a brownish red. The plastrum is slightly tinted with a similar hue, but the three crests formed by the 

 hip-pieces are, in virtue of their thickness of a deep colour. These are the red lines of Gras ^ Raspail and others. The 

 posterior legs have but a thin case of chytine, and are less deeply coloured. The coloured covering of the head and legs 

 contrasts very strongly with the yellowish white of the body of the animal. '*' 



,,The posterior legs spring from the posterior part of the thoracic segment of the animal, two on each side: they are 



