92 M. O. Stiobelt on the Anatomy and 



transverse grooves* In the interior of the space enclosed by 

 the above two membranes we easily observe a number of 

 pale -coloured blood-corpuscles furnished with a thin membrane 

 and a distinct nucleus. They are round and have a diameter 

 of about 0*013 millim. As already stated, the vesicular organ 

 is darker and also furnished with thicker walls than the 

 appendages of the tube. It also presents two membranes 

 an inner one containing fine granules, and an outer one perfo- 

 rated by small roundish apertures. In its interior also there 

 are numerous blood-corpuscles. 



As regards the mode of circulation of the blood in the body 

 of our animal, the following statement may be made. The 

 movement of the dorsal vessel takes place, as in all insects, 

 from the posterior extremity of the tube. The muscles sur- 

 rounding the vesicular organ, which is an analogue of the 

 heart in the higher animals, contract, and by this means the 

 organ becomes narrowed, and the blood which is pressed out 

 of it, in part directly, but for the most part into the tube, 

 passes out of the latter into its appendages, and comes into con- 

 tact with the organs of the body through their apertures. 

 That other muscles may not cooperate in this movement is by 

 no means certain. . When the tension of the muscles ceases 

 again, the vesicle enlarges to its original size, and the blood 

 goes back into it by the same way that it issued. 



The whole circulatory apparatus is accompanied by the 

 finest ramifications of the tracheae, which convey the oxygen 

 of the air to the blood. 



Finally, I have to confirm an opinion of Landois's, which 

 he has expressed with regard to the dorsal vessel. In his 

 i Anatomy of Pediculus vestimenti 1 (24, p. 40) , he says : 

 " Hitherto my endeavours to prepare the dorsal vessel of the 

 body-louse have been without result, which is explicable by 

 the certainly extraordinary delicacy of the organ. Neverthe- 

 less we cannot but ascribe a dorsal vessel to the insect. In 

 favour of the existence of such an organ we have not only the 

 analogy of other insects, but the connecting tubes of the 

 ovaries and testes also indicate it. With regard to these I 

 have convinced myself that the tips both of the strings of ova 

 and of the testes give origin to vessels which possess very 

 fine transversely striated muscular fibres, of which 1 could espe- 

 cially recognize circular ones, perhaps because they lie outer- 

 most. It is these vessels, as we know from insects that 

 have been investigated, which stand in connexion with the 

 dorsal vessel. From the existence of these, therefore, we may 

 deduce the presence of a dorsal vessel." This conclusion, 

 arrived at by Landois, is perfectly correct. I have been 





