THE EVIDENCE OF THE RESPIRATORY APPARATUS 161 
THE RESPIRATORY CHAMBER OF AMMOCCETES. 
In order to make clear the nature of the branchial segments in 
Ammoccetes, I have divided the head-part of the animal by means of 
a longitudinal horizontal section into halves—ventral and dorsal— 
as shown in Figs. 63 and 64. These figures are each a combination 
of a section and a solid drawing. The animal was slit open by a 
longitudinal section in the neighbourhood of the gill-slits, and each 
half was slightly flattened out, so as to expose the ventral and dorsal 
internal surfaces respectively. The structures in the cut surface were 
drawn from one of a series of horizontal longitudinal sections taken 
through the head of the animal. These figures show that the head-region 
of Ammoccetes consists of two chambers, the contents of which are 
different. In front, an oral or stomodeal chamber, which contains the 
velum and tentacles, is enclosed by the upper and lower lips, and was 
originally separated by a septum from the larger respiratory chamber, 
which contains the separate pairs of branchie. <A glance at the two 
drawings shows clearly that Rathke’s original description of this 
chamber is the natural one, for he at that time, looking upon Ammo- 
cetes branchialis asa separate species, described the branchial chamber 
as containing a series of paired gills, with the gill-openings between 
consecutive gills. His branchial unit or gill, therefore, was repre- 
sented by each of the so-called diaphragms, which, as seen in Figs, 63, 
64, are all exactly alike, except the first and the last. Any one of 
these is represented in section in Fig. 65, and represents a branchial 
unit in Rathke’s view and inmine. Clearly, it may be described as a 
branchial appendage which projects into an open pharyngeal chamber, 
so that the series of such appendages divides the chamber into a 
series of compartments, each of which communicates with the exterior 
by means of a gill-slit, and with each other by means of the open 
space between opposing appendages. 
Each of these appendages possesses its own cartilaginous bar 
(Br. cart.), as explaimed in Chapter IIT. ; each possesses its own bran- 
chial or visceral muscles (coloured blue in Figs. 63 and 64), separated 
absolutely from the longitudinal somatic muscles (coloured dark 
red in Figs. 63 and 64) by a space (Sp.) containing blood and 
peculiar fat-cells, etc. Each possesses its own afferent branchial 
blood-vessel from the ventral aorta, and its own efferent vessel to 
the dorsal aorta (Fig. 65, a. br. and v. br.). Hach possesses its own 
M 
