76 M. A. Vayssitre on the 
plates of much smaller dimensions, presenting numerous 
digitations on their margins. The respiratory functions 
being specially devolved upon these organs, it is a matter of 
course that they do not present the same consistence as the 
large plates; and their external envelope is even very delicate 
in the digitate parts. 
While we have in this genus a very marked localization of . 
the tracheo-branchial organs, such as is not observed in the 
commonest Ephemerina, we shall find in the next type a still 
greater localization, which has induced a concentration of 
various regions of the body. In fact in Betisca (fig. 2) we only 
find five pairs of respiratory organs, all of which fulfil the 
physiological function devolved upon them. ‘These organs 
present the appearance of very delicate plates with more or less 
digitate margins. ‘These plates are not protected, asin Canis, 
by one of their number modified for the purpose, but by a 
considerable development of the dorsal part of the thoracic 
integuments, which forms a prolongation reaching nearly to 
the sixth segment, thus giving origin to a sort of large cara- 
pace, under the shelter of which the respiratory apparatus 
performs its functions without running the risk of being 
damaged by contact with surrounding bodies. The move- 
ments executed in these tracheo-branchial plates give rise to 
a continuous current of water in this great cavity, which is 
directly in communication with the exterior at its posterior 
part and sides. ‘The animal can even slightly raise this 
carapace, at the same time causing its abdomen to execute a 
movement in the opposite direction, and thus facilitate the 
introduction of the surrounding liquid. 
There is no doubt that an equally great concentration of 
the various systems of the organism, and especially of the 
nervous system, is in correlation with this considerable locali- 
zation of the respiratory apparatus, as also with the coalescence 
of the thoracic segments. 
Let us return now to the examination of the same organs in 
Prosopistoma punctifrons. We observe an equally complete 
localization of the respiratory organs; but here the general 
concentration is more strongly marked (figs. 3, 4). 
In the first place, the first thoracic segment or prothorax, 
which in the preceding Ephemerine (Betisca) was still some- 
what distinct, is intimately soldered to the mesothorax 
without showing any traces of the union at the dorsal surface. 
The carapace formed by the prolongation of the dorsal 
integuments of the thorax cannot, in Prosopistoma, perform 
any movements, seeing that it is soldered to the neighbouring 
integuments by its lateral and posterior margins. ‘lhe cavity 
