‘THE HISTOLOGICAL STRUCTURE OF THE GILLS, ETC. 95 
thickness was varied from 3.3 m.'to 20 m. but those from 
6.6 m. were most satisfactory and this thickness was used 
in most of the work. After fixing to the slides the sections 
were de-colorized by 70% acid alcohol in a Petri dish under 
observation with the microscope:. When the decolorization 
had proceeded far enough the slides were transferred to dis-’ 
tilled water to which had been added a drop of ammonium 
hydroxide solution to neutralize the action of the acid during 
the time taken in transferring the slides from the acid alcohol 
to the water. After washing in this way the stain gave ex- 
cellent results. The slides were then carried up through the 
' alcohols and xylol and covered with balsam and cover glass 
in the usual manner. This method of decolorizing sections 
stained in bulk with haematoxylin and washing in distilled 
water to which a trace of alkali has been added, was found 
to be much more satisfactory than the use of ordinary tap 
weter since the sections were entirely free fro mdirt. 
The sections were cut in the three different planes of the 
gill, that is vertical, longitudinal-lateral, and longitudinal- 
transverse, the longitudinal-lateral section was only possible 
with small gills and even then was not very satisfactory since 
the sections were so large that complete serial sections were 
difficult to obtain. 
III.—CONCLUSIONS AND RESULTS. 
The simplest structure of the gills as was previously noted 
by Ortmann (4) is always found in the gills of the male. 
Figure 1 is a cross section of an outer male gill and shows the 
comparatively few filaments or septa between the interlamel- 
lar junctions. In figure 2 an innner female gill is shown and 
the greater distance between the inter-lamellar junctions, as 
measured by the number of septa between them, is the striking 
thing noticed. The outer lamella of the inner gill is also con- 
vex instead of being parallel to the inner lamella as it is in 
the other gill. . The effect of these differences between the male 
and femal gills is to give a much greater size to the water 
tubes in the tatter which is of course of great value during 
the period of incubation of the glochidia. 
The sub-filamentar outgrowths are large and abundant so 
