general cavity of the gill and contains blood (B.C.). The blood cavity is bounded by a thin wall
which lies immediately within the hypodermis (W.b.c.).
In the special portion of the gill there are no pillars but the blood-cavity is penetrated
by a system of branching canals. This system is in fact a single much divided cavity, tree-like in
form, and communicating with the outside of the gill through an opening (Op.). In other words, it
is morphologically an infold of the wall of the gill.
It is evident that this tree-like structure is a section of the corpus album.
The tree, as a whole, divides that portion of the blood-cavity of the gill in which it lies
into numerous small communicating spaces, containing blood (B.sp.). But at the base of the tree
and surrounded externally by that portion of the chitinous wall which is sculptured into furrows
that contain air is a large space or chamber (B.ch.). The quantity of blood contained in this
chamber gives the appearance of an accumulation of blood corpuscles, referred to above.
Minute I n t e r n a l Anatom y. The cellular structure of the hypoderm is indicated only
by the nuclei, no cell-walls being visible. The layer varies in thickness in different parts of the
gill. It is best developed in the general part where the nuclei are large and surrounded with cell-
protoplasm. In the special part, along the two walls of chitine, the layer becomes thinner and the
nuclei are flattened. Along the infolded portion of the chitinous wall, forming the tree, the cell-
protoplasm is greatly reduced and the layer becomes very thin, except where the nuclei occur (Hy.
Nu.). The nuclei are everywhere marked by clearly defined boundaries and are highly granular.
The pillars consist of hypodermic tissue and at their ends are structurally continuous with the
respective dorsal and ventral hypodermic layers. They contain one or more nuclei which are
elongated in the direction of the axis of the pillar.
The blood cavity occupies the whole space within the hypodermic layer and is bounded by
a thin wall lying contiguous to the hypoderm and conforming to its irregularities (W.b.c.). In this
wall at long intervals lie elongated nuclei. I infer, from the relations of this wall that it is of
mesodermic origin.1
1 S inc e th e p re s e n t tre a tis e was w r itte n m y a tte n tio n h a s b e en c a lled b y P ro fe s so r C h u n to th e w o rk o f L e i c h -
m a n n , B e i t r a g e z n r N a t u r g e s c h i c h t e d e r I s o p o d e n , p ublished in th e B ib lio th e c a zoologica, H e ft 10.
Cassel 1891. I t a p p e a rs from th is work th a t th e brood-sa cks o f th e Iso p o d a a r e in th e ir g e n e ra l fe a tu re s homologons
w ith th e gills. As shown b y th is a u th o r th e bro o d -sa ck s a r is e a s folds o f th e h y p o d e rm a n d a t a n e a r ly s ta g e o f d ev e lo
pm en t th e hy p o d e rm c e lls become g rouped in such a w ay a s to le a v e a n e t-w o rk o f sp a c e s b e tw e en th e o p p o site walls
o f th e sa ck . A cross-se c tion o f th e b rood-lame lla o f A s e l lu s a q u a tio n s (a figure o f which i s g iv en ) ta k e n a t th is s tag e
o f d eve lopment shows a n id e n tity in s tru c tu r a l p la n w i th t h a t o f th e g ills o f th e Oniscidae, in th e c a ses whe re th e l a t t e r
h a v e u n de rgone no sp e c ia l modifications in a d a p ta tio n to a ir -b r e a th in g , a s in th e l a s t th r e e p a ir s o f o u te r g ills o f P o r c eU io
s c a b e r . According to L e i c h m a n n , th e n e t-w o rk o f sp a c e s in th e b ro o d -lam e lla e wh ich , a s in th e g ills, co n ta in blood, a re
lacunose. He figure s th e s e sp a c e s a s bounded b y a c le a rly defined lin e , b u t h e re g a rd s th is lin e a s m e re ly th e b o u n d a ry
o f th e hy p o d e rm ic wa lls a n d p illa rs . I n m y d e s c rip tio n o f th e g ills , a s above g iv en , I h a v e co n sid e red th is b o u n d a ry
lin e a s forming a definite w a ll su rro u n d in g th e blood-spaces a n d p ro b a b ly o f mesodermic o rigin. I was in c lin ed to th is
view from th e f a c t t h a t nuc le i occur, th o u g h v e ry sp a rs e ly , a lo n g th e s e lin e s w h e re th e y im m ed ia te ly ad jo in th e h y p o dermic
wall.
A c cording to m y co nc eption a la y e r o f mesodermic o rigin which bounds th e b o d y -c av ity e x te n d s in to th e gills
an d forms a b o u n d a ry w a ll o f th e blood-spaces o f th e gills. On th e o th e r h a n d L e ic h m a n n finds in th e brood-lamellae
no such la y e r b u t conside rs t h a t th e sp a c e s a r e la cu n a e a n d t h a t th e blood lie s im m ed ia te ly in c o n ta c t w ith th e h y p o derm.
T h e c le a rly defined lin e su rro u n d in g th e spa c e s is th e in n e r m a rg in a l lin e o f th e hyp o d e rm ic tis su e . L e ic h m a n n
sp e a k s o f th e “in n e r e C h itinlam e lle “ .
I n view o f th is d is c rep an cy I would le a v e a s u n d e te rm in ed in th e p r e s e n t w o rk th e n a tu r e o f th e wa ll in question.
I h o p e a t a fu tu re tim e , th ro u g h th e s tu d y o f th e em bryology o f th e g ill, to d e te rm in e th e p o in t finally.
Where the hypodermic pillars intersect the blood cavity the dorsal and ventral boundary
walls of the cavity become continuous, forming sheaths which surround the pillars. The ramifications
of the tree are also accompanied by folds of the boundary wall of the blood cavity (Fig. 6, W.Bc.).
In this situation the wall becomes very thin but it may be seen as a clearly defined boundary of
the communicating blood spaces formed by the divisions of the tree.
The structure of the tree is well seen in sections taken parallel with the faces of the gill
(Fig. 7). The opening through which the ramified cavities of the tree communicate with the outside,
here appears as a rather narrow passage and the chitinous walls along the passage are sculptured
in the same manner as the specially modified portion of the exterior chitinous wall (Op. tr.;
Gr. ch.). The grooved structure of the exterior wall is in fact continued in the infolded portion
and gradually becomes simplified to the plain wall of the branches of the tree. The passage widens
into a shallow bowl-shaped cavity from which pass out in radiate arrangment the tubular cavities
of the primary branches of the tree (Br. tr.). These divide repeatedly and the final terminations
approach closely to the walls of the gill (Fig. 6). The mode of branching does not appear to conform
to any definite law, excepting that the divisions lie at approximately uniform angles with one
another. Where a division occurs the size of the tubules is reduced only to a moderate extent,
so that the final branches have a relatively large diameter. The branches terminate in rounded ends.
The walls of the tubules are thin and show no markings. Lying outside of the chitinous
layer is the likewise thin hypodermic layer. Outside of this and closely associated with it is the
boundary wall of the blood cavity (Fig. 6; Ch., Hy., W.b.ci). Thus every portion of the wall of
the tree is composed of three very thin layers. The hypodermic nuclei occur at frequent intervals.
It is seen from the above that the study of the internal anatomy of the gill establishes
the conclusions of those authors quoted above who found the corpus album to be a branching, treelike
cavity communicating with the exterior through an opening. The conception of D u v e r n o y and
L e r e b o u l l e t that the tree is morphologically an inward folding and division of the inner wall of
the gill is verified. Incidentally to the study of the anatomy it has been shown, as has been hitherto
generally believed, that the cavity contains air.
The P h y s io lo g y o f th e G ill. In considering the physiology of the parts described
above it may first be noted that the gill as a whole has no function that is merely relative to the
inner gills. The name gill-covers usually given by authors to the outer branches of the appendages
is not applicable to the first and second pairs, since the three pairs of inner gills lie within the
last three pairs of the outer branches.
As regards the use of names it may be remarked that it seems most suitable to employ
simply the terms inner and outer gills, using them in their morphological values.
The fu n c tio n o f th e tr e e and th e grooves. It is evident that the gill of PorceUio
scaber is a structure adapted to bringing the blood into relation with ah’. Two different and independent
anatomical structures are employed in order to secure a ready exposure of the two media
to each other. These are first, the net-work of furrows in the chitine at the exterior of the gill
and second, the infolded portion of the chitinous wall, forming the internal tree.
In considering the physiological significance of the details of structure of these two parts
it is helpful to bear in mind the mechanical conditions requisite to securing the general end. These
are first, an extension of the wall interposed between the two media, in order to afford a large