perfect State of Prosopistoma punctifrons. 17 
which it forms, and which we shall designate the respiratory 
chamber, is put into communication with the exterior only by 
three apertures—one dorsal and median, two ventral and 
lateral. ‘The water destined for respiration penetrates by the 
ventral apertures (fig. 4, v,v'), and, after having aerated the 
tracheo-branchiz, issues by the dorsai opening. This current 
of water is produced by the movements of the first two pairs of 
respiratory organs. ‘The two very elongated plates of the 
first pair are destined to cause the entrance of the liquid by 
the ventral apertures, while the two large quadrangular 
plates which form the second pair expel it by the dorsal 
aperture. The other three pairs of respiratory plates, which 
can only be detected by removing the quadrangular lamine, 
present numerous digitations, which facilitate the interchange 
of gases *. 
Of course the nervous system participates in this general 
concentration of the mteguments of Prosopistoma. It con- 
sists :—(1) of a pair of cerebroid ganglia, closely applied to one 
another, and innervating the organs of the senses (eyes, ocelli, 
and antenne) ; (2) of a single subcesophageal or pharyngeal 
ganglion, which furnishes numerous nerves to the various 
regions of the head; and (8) of a very voluminous thoracic 
ganglion, which represents of itself the whole ganglionic 
chain, and consequently sends forth nervous trunks to all 
parts of the thorax and abdomen. Upon the contour of this 
nervous mass we may sometimes observe swellings indicating 
the original existence of three pairs of thoracic ganglia and of 
one abdominal ganglion, 
I think that the details I have just given as to the organi- 
zation of Prosopistoma in the nymphal state, as well as the 
resemblance that can be set up between this insect and the 
nymphs of the two most nearly related genera, will be of 
service towards the ready comprehension of the importance of 
the modifications which I have ascertained to occur in the 
perfect insect, and to justify the uncertainty in which, like 
many other naturalists, I found myself as to the final state of 
this Arthropod. , 
~ It would appear that the metamorphoses of Prosopistoma 
into the subimago and imago must take place in the course of 
June; at least, as I have stated in my note to the Academy, 
this is the period at which I ascertained the fact, and a period 
nearly the same as that at which many Ephemerina are 
metamorphosed. 
* It is very probable that the insect at its escape from the egg does not 
possess tracheo-branchiz, and that respiration is effected for a certain 
time through the skin of the general surface of the body; the carapace 
must be more or less incomplete during this larvular phase. 
