NATURAL HISTORY OF THE GENUS DERO. 
97 
so great that it can only be kept in the field of a power too low 
for needful details to be made out. 
For general observation the method which. I have found 
most suitable is to transfer the worms to a live-trough, with 
a sufficient depth of mud for them to form their tubes 
(about 5 inch), when they may be observed under perfectly 
natural conditions ; the hinder end of the worm, carrying the 
respiratory apparatus, being kept protruded upward, whilst the 
head is occupied below in ingesting the mud which forms the 
food of these creatures. If the tubes have been formed amid 
vegetable debris , the best plan is to secure a portion in the 
compressorium under slight pressure, or in a small zoophyte- 
trough, when powers as high as Zeiss’s B B or an English f-in. 
may easily be employed. To make out the histological details, 
nothing is better than the cotton-wool trap used for wandering 
Itotifera; with this and a judiciously regulated pressure, a -A-in. 
objective may be safely used. The form of compressorium 
adopted by me is that known as Beck’s parallel compressorium, 
and I have found nothing to equal it for the facilities it offers of 
increasing or diminishing pressure without removal, and of 
viewing an object on both sides. 
General Characters. 
In general outline the species of Dero closely resemble their 
relatives of the genus Nais. The following marked differences, 
however, obtain 
1 . They are destitute of eyes. 
2. They are furnished with decidedly red blood. 
3. The perivisceral fluid is devoid of corpuscles. 
4. They inhabit fixed tubes. 
5. They possess a highly specialized respiratory organ on the 
last segment of the body. 
The general form of the body is more or less cylindrical, the 
head being obtusely pointed. The thickness gradually increases 
from the head for about two fifths of the length of the worm, after 
which it diminishes gradually again, being narrowest in the last 
segment but one. 
As in Nais, the mouth-segment is destitute of organs of 
motion, whilst the four following have them only on the ventral 
surface *. 
* Except Bern furcata, of which see description. 
