126 DR. L. A. BORRADAILE ON THE 
than the rest o£ the organisation of that animal. In its widest sense, the 
term may be applied to the whole of the very large space that lies between 
the flank of the body and the over-arching fold of the carapace known as the 
branchiostegite, which encloses the chamber aboA'e, and without, and partly 
below. The flank itself constitutes the inner wall of the chamber. Of the 
two layers which compose the branchiostegal fold, the outer is hard and 
calcified, that which is towards the chamber is membranous. In their hinder 
region the two layers are not widely separated, and there are between them 
only blood-vessels and connective tissue; but anteriorly certain viscera 
intrude into the upper part of the fold. The hard layer of the branchio- 
steo-ite lies at first almost horizontal, so as to form a roof above the chamber 
and the intruding viscera. It is then turned downwards at an angle, which 
in the hinder region is obtuse but becomes more and more acute as it is 
followed forwards, to form a wall which curves inwards till it reaches the 
flank of the body above the bases of the limbs. As the angle at which this 
wall joins the roof becomes more acute, the wall faces more downwards, till 
in the fore part of the body a portion of it becomes horizontal; there it forms 
a floor to the chamber. The shape of the chamber is of course determined 
not by the contour of the outer layer of the branchiostegite, but by that of the 
inner. This layer makes in the hinder part of the thorax a continuous curve 
from above downwards, constituting an arched inner roof to the chamber, 
which has here no floor properly so called. In front, however, where the 
outer wall becomes horizontal, the inner wall turns inward at an angle to line 
the floor of the chamber. In the thoracic part of the latter, the floor under- 
lies the overhanoing part of certain gills which project considerably above 
the bases of the limbs. In the head, it closes from below the exhalent 
chamber shortly to be described. A number of " dorso-ventral muscles" 
emible the membranous roof to be raised or lowered. 
The chamber is sharply divided into two parts — a true '■ gill-chamber " in 
the thoracic region, and an "exhalent passage" or " prebranchial chamber" 
in the cephalic region. Of these divisions the gill-chamber is much the larger 
in every dimension. Its inner wall is battered back so as to face upwards 
as well as outwards, and is brittle, though thin, and composed of broad ribs 
— the so-called " epimera " — one to each limb from the last leg to the second 
niaxilliped inclusive. That which lies above the cheliped is larger than the 
rest and prominent. Those behind it face a little backwards and those in 
front face forwards, so that the whole wall is convex outwards, forming a 
low, roughly half-conical mound. In front, the chamber narrows rapidly, 
its membranous roof at the same time falling steeply to the hinder opening 
of the exhalent pacsage, where the roof is upheld, and the opening maintained, 
by an arching, calcified sclerite. Here the roof turns forward as that of the 
exhalent passage. This is a shallow chamber which diminishes in width as 
the carapace narrows forwards. Its inner wall has become merged in the 
