to me to warrant the inference that it functions directly as an organ of respiration of air. It is
true that as the blood circulates through the gill it is separated from the external atmosphere by
the rather thick ventral wall of chitine. But through the movements of the gill, taking place as
described for Porcettio air is constantly brought into relation with the inner surface where the
chitinous wall is thin. It seems probable that the conditions requisite for an exchange of gases
between the blood and the air are here satisfied.
In addition to the respiratory function the outer gills serve as covers for the inner gills.
The In n e r Grills. The last three pairs of inner gills of IAgidium are in all respects
identical in general structure and relations with those of Porcettio scaler. The first two pairs, as
already stated, p. 6, are rudimentary.
Oniscus murarius Cuvier.
In the genus Oniscus the anatomy of the outer gills differs greatly from that of the genera
thus far described. They lack any trachea-like structures but in their place possess special modifications
adapted to the same functional end.
The first right outer gill of the male, examined on the inner or dorsal surface is represented
in Fig. 11. It is seen that the gill is somewhat triangular in general outline, with rounded corners.
The articulation of the gill with the basal joint (art) is situated on the anterior and outer side.
The inner side lies parallel with the middle line of the body and is nearly straight. The side
lying outwardly and posteriorly is deeply indented, the angle thus formed dividing this portion of
the gill into two unequal lobes. The outer of these lobes is the portion of the gill specially modified
in adaptation to the respiration of air. Viewed from above it is seen to constitute the outer-
posterior portion of the gill and to comprise about one-third of the whole surface. Its margin is
semi-circular in shape and its line of union with the general part of the gill is slightly convex
toward that part. Along this line is situated a broad channel which, as will be shown below, conducts
blood, after being aerated in the gill, back to the body and may be designated the main
excurrent channel. Communicating with this channel is a series of small channels, arranged in a
somewhat radiate manner, and occupying the whole area of the special part of the gill. Another
broad channel is situated along the anterior and inner margin of the gill. As will be shown below,
this receives blood from the body and it may be designated the main incurrent channel.
The bonndaries of all the blood-channels are marked by rows of dots. Such dots are
also seen scattered over the middle region of the gill, more or less aggregated into groups.
The chitinous covering of the gill is seen as a somewhat broad line along the entire margin
of the gill. It is provided with stiff hairs on the outer and posterior border.
If the gill of a living animal is examined with a hand lens the special part presents the
appearance of containing air. This may be better seen by placing a specimen in water under a
cover-glass and examining with a low power of the microscope. Corresponding with the radiate
channels above noted are seen a series of wave-like elevations and depressions which under reflected
light appear silver-white and glistening. Most of the waves divide toward the margin of the gill,
becoming Y-shaped, thus giving rise to the radiate arrangement of the parts, as a whole. Underneath
the wave-like elevations the corpuscles of the blood may be seen to circulate.
This appearance lasts but a short time. The waves gradually become flatter and narrower
and in the course of 10. minutes pass away altogether. The shining white aspect gives place to
the ordinary appearance of the rest of the gill. If the animal is then restored to air the original
appearance gradually comes back and in about 30 minutes is fully regained.
Minute E x t e r n a l A natom y. Under high' magnification the chitinous wall shows
superficial markings forming a pattern-work similar to that already described for Porcettio scaber.
The dots, as in the species already described are the nuclei of the hypodermic layer. The pillars
of the hypoderm come into view by focussing at different levels. Along the margins of the blood-
channels these pillars are arranged in rows, forming a kind of broken wall to the blood-channels.
Within the cavity of the gill blood-corpuscles may be seen. They consist of the two forms
of cells already described for Porcettio scaber.
I n t e r n a l A na tom y . Fig. 12 represents a cross-section of the gill taken in the position
of the line ss', Fig. 11. It is seen that the gill is of moderate thickness throughout and that the
outer fourth (right side of the figure) is much thinner than the rest. This thin portion of the gill
in the special part; that is to say, the part specially modified for respiration and containing in the
living animal, as noted above, air. The remaining thicker portion constitutes what we have designated
above the general part of the gill.
The chitinous wall of the gill varies in thickness in different regions. It shows its greatest
development on the dorsal side of the general part, where the chitine makes up nearly one-third
of the breadth of the gill. On the ventral side it is everywhere thin, but less so in the general
than in the special part. In this latter part the chitinous wall is thrown into elevations and depressions
(see also Fig. 13).
The hypoderm consists of a continuous layer of tissue lying immediately within the chitine
and, in addition, the bridges or pillars of tissue referred to above. In general the hypoderm layer
varies in thickness with the chitine. The nuclei are conspicuous and vary widely in respect to size,
form and position. Where the layer is thick the nuclei are large and tend to be elongated in form
and arranged with the long axis at right angles to the wall of the gill. Where the layer is thin
the nuclei are flattened and lie parallel with the wall of the gill. In the special portion of the gill
where the hypodermic layer is very thin the nuclei appeal* to be withdrawn from the layer and to
be associated with the tissue of the pillars (Fig. 13, Hy. pi.). It is seen further that the layers of
hypoderm between the bases of the pillars are arched outwards. In other words, where the pillars
occur the opposite dorsal and ventral hypodermic layers are drawn inward, thus being thrown into
the form of waves. On this account the chitinous wall of this part of the gill has, as noted above,
an undulated form.
The s tr u c tu r e and r e la t io n s of th e g en e ra l p a r t o f th e g ill. In Oniscus the
general cavity of the gill is not a simple cavity as in Porcettio but is separated into a number of
chambers by means of partitions lying parallel to . the walls of the gill.
In the general part of the gill there are two of these chambers, an inner or dorsal chamber
and an outer or ventral one (Fig. 12, d.b.c., v.b.c.). They contain blood. Each chamber is bounded
above and below by a thin wall. It results from this that the cavities of the two chambers are
separated by a double wall (Pt.). This appears as two fine parallel lines between which is a very
narrow space. Lying in this space and occurring at frequent intervals are elongated nuclei. The
outer walls of each chamber lie immediately within the hypodermic layers on the respective dorsal
Bibliotheoa zoologioo. Heft 86. 4