484 THE BLOOD-CORPUSCLES OF THE ANNELIDES. 



The words I have italicised are quoted from my book ; but I did 

 not speak of blood, but of blood-vessels, as combining with another 

 blood-carrying cavity. This, however, is mere verbal criticism. 

 "What are the facts of the case? Branchiobdella is an animal which, 

 so far as I know, never exceeds \ inch in length, and which, for a 

 satisfactory examination of its internal organs, needs to be rendered 

 transparent by lying some days in acetic acid and subsequently in 

 glycerine. When examined whilst alive under the microscope, 

 Dr. Dorner (' Zeitschrift Wiss. Zoologie,' xv. p. 479, 1865) tells us 

 that the observation of its circulation is made exceptionally difficult 

 by the fact that the dorsal vessel in the posterior part of its length 

 is very closely apposed to the digestive tract, and whilst its walls 

 are in this non-contractile portion of its length extremely thin and 

 transparent, the digestive tract, on the other hand, is dark coloured. 

 In the larger of the two species, Branchiobdella parasita, it is true, 

 Dr. Dorner says the intestine has a less dark colouration, and 

 that it is possible in it to make out clearly the course of the 

 dorsal vessel, especially if the animal is kept a long time under the 

 covering glass, and subjected to gentle pressure. The pressure, 

 I will remark, must be very gentle indeed, if it is not to introduce 

 a source of fallacy into such an investigation as this ; and this con- 

 sideration may well make us hesitate before accepting in their 

 entirety, and in the strict sense which the author himself con- 

 templated, the following words of Dr. Dorner (p. 499 1. c.) : — 



1 Das gesammte Blutgefass-system des Korpers steht so mit im unmittelbaren 

 Zusainmenhange, und stellt einen vollstandigen Kreislauf dar.' 



I may illustrate this matter further by referring to the disputes 

 which were raised in former days as to the existence or non-exist- 

 ence of an orifice of communication between the anterior or 

 exteriorly opening sac of the organ of Bojanus and the secretory 

 part of the organ even in animals of the size of our fresh-water 

 mussels, where the difficulties of being quite sure as to one's ground 

 were by no means so great as in this case. Branchiobdella , how- 

 ever, has not, so far as I have been able to find, been as yet met 

 with in the parts of the United Kingdom with which I am ac- 

 quainted, illustrating therein the well-known natural history law 

 of the partial, and to us often inexplicable, distribution of parasitic 

 organisms, to the life of which, and consequently to the diseases 

 produced by which, other conditions are often necessary besides 



