NEW POINTS IN THE STRUCTURE OF AMPHIOXUS. 259 
form of the pharynx, the structure ofits walls, and median 
folds which 1 cannot enter into without figures. 
2. Boundaries of the atrial chamber.—Anteriorly the atrial 
chamber commences with the pharynx ; posteriorly it is con- 
tinued far beyond the atrial pore, as far back as the anus. 
But it is to be noted that after the atrial pore is passed the 
atrial chamber (still lined by its pigmented atrial tunic) 
becomes very much diminished in size and pushed on one 
side (viz., the right) by the enlargement of the true peri- 
visceral space or celom. In the region beyond the atrial 
pore the atrium, in transverse section, is reduced to a space 
one third the size (or less) of the ccelom in the same section. 
3. The true perivisceral space or celom.—The space which 
thus enlarges posteriorly is represented throughout the course 
of the intestine, and has been correctly figured by Stieda 
as to general features in his Plate I, fig. 7 and Plate II, fig. 
8. The space in question is perivisceral, extending round the 
intestine. It is traversed by certain blood-vessels and itself 
contains a coagulable fluid. It lies between two pairs of 
membranes which are readily distinguished. The outermost 
is formed by the visceral reflection of the pigmented epithe- 
lium of the atrial tunic and a fibrous supporting layer placed 
beneath. Then follows the space. Forming its intestinal 
boundary we find another fibrous layer, and internal to this 
we have the very thick intestinal epithelium. 
This space is continuous with the uppermost of the series 
of chambers lying between membranous outgrowths of the 
pharyngeal bars, by means of which the upper part of the per- 
forated pharynx is attached to the walls (epipleura) of the 
upper part of the atrial chamber. It is also continuous with 
the narrow space between any two folds attached to a single 
bar (see below). Stieda (p in figs. 3, 4, Plate I) correctly 
assigns the uppermost of these chambers to the pleuroperi- 
toneal cavity (as distinct from the atrial chamber). The 
ceelom in the region of the pharynx becomes very much: 
reduced, but it is obvious enough between the coecum and 
pharynx in a transverse section. It can be traced as a 
very much reduced lymph-space in mid-line both above 
and below the pharynx in transverse sections. Figures are 
necessary to render its exact position in each region intel- 
ligible. It is further traceable beneath the parietal (epi- 
pleural) layer of the atrial tunic, being dilated in this position 
at certain points, especially in the region of the body pos- 
terior to the atrial pore. ‘The genitalia are developed in 
epipleural dilatations of the celom. 
Pharyngo-pleural septa, lateral attachments, or suspensory 
