58 THE NORTHERN MICROSCOPIST. 

sight of this phenomenon through the microscope is one of con- 
siderable interest. The protecting plates are first raised, when the 
four pairs underneath are immediately seen in their proper posi- 
tions in rapid and rhythmical motion, the long fringes lashing the 
water and creating currents that send the floating particles of 
matter eddying in the most tortuous courses; when, after con- 
tinuing in action for a minute or two, the protecting plates sud- — 
denly descend upon the rest, and, instantly pressing them out 
of sight, leave nothing visible but the upper surfaces of the large 
plates in perfect repose. Those who delight in witnessing such 
sights as the circulation of the blood, as seen in the vascular plexus 
of the frog’s web, or the beautiful contortions of the tentaculated 
crowns of the polyzoa, may perhaps find a new sensation in wit- 
nessing the branchial performance of the nymph of the genus 
Ceenis. 
An interesting discovery in relation to the circulation of the 
blood in the Ephemeride has comparatively recently been made 
by Herr O. Zimmermann, and recorded in the Zectschrift fur wis- 
senschaftliche Zoologie, for 1880. A note alluding to this discovery — 
will be found in the Journal of the Royal Microscopical Society 
for last year. It had been long known that the valves which 
separate the dorsal vessel into chambers open in an anterior direc- 
tion, the contractions of each chamber propelling the blood forward 
towards the head; but how this fluid not only entered the long 
caudal appendages, but was also maintained in active circulation 
through the greater portion of their length, was a problem which 
entomologists had not satisfactorily solved. The German investi- 
gator has found in some of the smaller larvee, such as Cloéon and 
Ceenis, a provision for this purpose. In the last chamber the 
valve, instead of opening forwards, opens backwards, so that when 
that chamber contracts the blood is forced backwards into the 
central vessel of each seta, from which it finds its way through a 
long oval aperture into the peri-vascular cavity, and is then pro- 
pelled forwards again by what physiologists call the vs a ¢ergo of 
the current. The walls of these appendages being very thin, 
allowing the return current of the blood to come very close to the 
surface, it is supposed that the circulation here has a respiratory 
function as well as a nutritive one. 
There is still much to be learned with regard to the aquatic 
forms of this family of insects. Some of the nymphs are imper- 
fectly known, especially in the early stages of their growth ; and 
any scientific microscopist who will trace the life-history of some of 
the rarer species from the egg to the imago, will not only throw 
| some light upon the development of the Ephemeridz, but may 
possibly be a contributor of important facts to the domain of 
| entomological physiology. 

