100 



MR. E. C. BOUSFIKLB 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 AulopJiorus 

 of Schmarda (25), yet in its full development it is present only in 

 the genus under notice. In all the Oligochaeta a strong inward 

 current is visible iu 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, w^hich, 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 PL 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 difi'ers in difl'erent 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. Eeference 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 wljich can 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 



