100 
MR. E. C. BOUSFIKLD ON THE 
Branchial Area and Circulatory System. 
The branchial apparatus is by far the most important character 
of Dero. Though it is found in a modified form in the Aulophorus 
of Schmarda (25), yet in its full development it is present only in 
the genus under notice. In all the Oligochseta a strong inward 
current is visible in the hinder part of the intestine, which no 
doubt subserves a respiratory purpose, as it commences at the 
point where the arterial system receives the blood from the 
venous ; and at this point, if nowhere else, both vessels run in 
close contact with, or are embedded in, the intestinal wall, so that 
the most favourable conditions for interchange are combined. In 
those forms which live and move free in the water, this is 
doubtless sufficient provision for their needs ; but in the case 
of Dero, which, though not fixed, is yet stationary, a special pro- 
vision seems required, such as is found in the branchial area. 
This is essentially, and in its simplest aspect, an opening-out 
of the hinder part of the intestine, supported by a layer of 
epidermis, the space between the two being occupied by muscular 
elements and blood-vessels. Figs. 4 and 5 in PI. IV. (the former 
taken from Perrier’s monograph, the latter from life) will show 
the general arrangement. In fig. 5, which gives the area in sec- 
tion, the relative proportions between its two constituents are 
shown by the shading, the dark portion being integumental, 
whilst the light shading indicates the continuation of the mucous 
membrane of the intestine, which supplies the respiratory 
element. The form of the area differs in different species ; but 
in all cases a number of branchial processes arise from the floor 
of it, their number being four and no more. This may seem 
a surprising statement, in view of the considerably greater 
number, eight pairs according to one writer already referred to, 
which have previously been described. Reference has already 
been made to the difficulty of determining the species of any 
given example of the genus, and similar difficulties, not overcome, 
have been the cause of the remarkable divergencies and dis- 
crepancies to be found in the descriptions heretofore given. 
These have obtained down to the latest period : thus both Semper 
and Leidy have described appearances which cau easily be re- 
produced, but which are not by any means normal. If an apology 
be needed for such an assertion, I hope it will be found in the 
fact that I have devoted myself almost exclusively to the study 
