INTERNAL AXATOMY OF S(^UIELA. 
409 
simultaneously their Imnina become invaded by numerous 
invaginations of their walls (two incipient invaginations are 
shown in Text-fig. S). These invaginations are formed at all 
points of the walls of the enlarged kidney and end-sac ami 
become so long, branched and closely compacted internally 
in the former that the originally spacious lumen is reduced in 
all parts to thin cracks lying between adjacent cell-layers 
(Text-fig. 4). The entire exterior of the gland is invested 
with two layers of squamous epitheliu!u, between which lies 
a division of the hmmocoele. The inner layer (I. Sq. Ep., 
4’'ext-fig. 4) only is closely applied to the surface of the entire 
gland, and becomes involved in all the invaginations just 
referred to. Careful inspection of well-stained sections shows 
('Fext-fig. 4), as might be inferred from the development of 
the gland just outlined, that the mass of the gland exhibits 
two kinds of spaces : (i) Spaces h^emocoelic in nature, and 
therefore morphologically outside the gland, lined by the 
s(|uamous epithelium, which originally covered the gland 
surface and which subsequently followed the invaginations of 
the gland wall ; and (ii) spaces devoid of a squamous epithe- 
lium, being solely bordered by the gland-cells, and represent- 
ing all that is left of the originally wide lumen of the gland. 
Thus, it will be seen that the narrow blood-space surrounding 
the gland penetrates by means of the invaginations described 
into the innermost parts of the mass of the organ, and in 
every part is only separated from the lumen of the gland by a 
thin squamous epithelium and the single layer of gland-cells 
which purify the blood. The squamous epithelium forming 
the outer wall (0. Sq. Ep.) of the haemocoele, and lying 
wholly on the periphery of the gland, lies for the most part in 
close contact with the inner squamous layer (/. Sq. Ep.), where 
this is situated at the periphery, and in my sections can only 
be distinguished from it at those points where the inner layer 
becomes invaginated to line the involutions of the gland wall, 
and so necessarily becomes separated (Text-fig. 4). 
In longitudinal sections of the gland and duct (PI. 28, fig. 
3), the three regions characteristic of Crustacean antennal 
