NATURAL HISTORY OF THE GENUS DERO. 



97 



so great tbat it can only be kept in tlie field of a power too low 

 for needful details to be made out. 



Eor 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 I 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 |-in. 

 may easily be employed. To make out the histological details, 

 nothing is better than the cotton-wool trap used for wandering 

 E/otifera ; with this and a judiciously regulated pressure, a ^V"^^* 

 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. Tliey 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 J)ero furcafa, of which see description. 



