114 Wisconsin Academy of Sciences , Arts and Letters. 
greater length, and possessing more facts than his predecessors, has adopted 
their theoretical views without modification. His latest contribution, how¬ 
ever, seems to show a tendency to depart from the standpoint held in his 
paper on polypody, a change of opinion attributable to his study of Steno - 
bothrus. 
The facts which have led to the assumption of the gill hypothesis are the 
following: 
1. The position of the pleuropodia on the pleurae near the insertion of 
the metathoracic legs could not fail to suggest the respiratory organs of the 
Crustacea. 
2. In some insect embryos the pleuropodia are shaped like lamellar gills. 
This is notably the case in Melolontha , the pleuropodia of which are so gill¬ 
like that Graber figures an Isopod side by side with the insect embryo for 
the sake of comparison. 
3. Blood has been observed to circulate in and out of the pleuropodia. 
That these facts are not sufficient to sustain the hypothesis, is shown by 
the following consideration: 
The pleuropodia of Melolontha certainly resemble the gills of certain 
Isopoda, but it is almost equally certain that the appendages of the 
cock chafer have departed from the original type of pleuropodium which 
is best seen in the Orthoptera [ Blatta , Stenobothrus]. As these organs 
were present after hatching, in ancestral forms, it follows that in the pre¬ 
cursors of Melolontha , they might have been balloon-shaped after the ani¬ 
mal’s escape from the egg. Such large, sack-shaped organs must necessarily 
become much flattened while they are confined to the narrow space be¬ 
tween the body-wall of the embryo and the egg envelopes. 
The cell-layer forming the walls of the pleuropodia in all cases where 
these organs are not rudimental, is considerably.thicker than the ectoderm 
of the appendages and body walls. Now, as a gill in ultimate analysis 
is merely a thin layer of cells separating the blood from the air, it becomes 
very difficult to understand why the comparatively thin layer of ectoderm 
cells forming the walls of the thoracic and cephalic appendages and the in¬ 
tegument in general should not constitute a much more efficient respiratory 
organ than the thick-walled pleuropodia. 
Blood has been seen to circulate in and out of the pleuropodia, but it also 
circulates in and out of the legs, mandibles, anal stylets, etc., in the same 
manner. 
The bulbous shape of the pleuropodia iri the majority of forms is unlike 
that of any known insect gills. The tracheal gills seen in the larval Ephe- 
meridse, Odonata, etc., are foliaceous or filamentous; while the protrusile, 
anal gills of such forms as the larval Eristalis are tubular. 
Lastly, appendages shaped like the pleuropodia of Meloe, Acilius,Zaitha 
and Cicada, could not have had a respiratory function. The hypothesis is 
therefore insufficient to cover all the facts and must be either restricted to 
a few doubtful cases or abandoned altogetner. 
