378 
B. EAY LANKESTER. 
The structure of the pharyngeal bars, and the number and 
character of the spaces contained in them, as shown in 
transverse section, also have been the subject of divergent 
and erroneous statements. 
The atrial cavity can be readily traced in sections of well- 
grown specimens, owing to the fact that the epiblastic epithe- 
lium by which it is lined, is loaded very often with brown 
pigment granules. For the purpose of tracing the atrial 
cavity, a specimen should be chosen which has the brown 
pigment well developed; it is more abundant in some indi- 
viduals than in others. The general limitation of the atrial 
cavity as seen in a transverse section about the twenty-seventh 
myotome, is shown in the diagrammatic figure given in 
PI. XXXYI. Other facts with regard to the atrial cavity are 
shown in the “ reconstructed ” dissection of PL XXXIV, 
fig- !• 
A curious fact with regard to the atrium (first described by 
Rolph) is the existence of a csecal prolongation of its cavity 
beyond the atriopore posteriorly. This atrial caecum pushes 
its way as a tapering blind sac into the perienteric coelomic 
space behind the atriopore, and occupies a position between 
the intestine and the musculature of the body wall. It 
reaches as far back as the anus, where it terminates blindly. 
It is represented in PI. XXXIV, fig. 1, for the first time as 
exposed in a simple dissection, Rolph’s and Langerhaus* 
figures showing it in transverse section. 
The enteric cavity of Amphioxus presents three main 
regions, viz. the pharynx, the intestine, and the caecum. 
Owing to the enclosure of the true original surface of a large 
part of the body by the atrial or epipleural folds, a misleading 
nomenclature is apt to be applied to the regions of the body 
thus enclosed ; we are led to overlook the fact that the wall of 
the perforated pharyngeal region, the wall of the caecal region, 
and the wall of the intestinal region as far as the atriopore, are 
not the proper walls of pharynx, caecum, and intestine, but in 
reality epidermis-clothed somatopleur or body wall, enclosing 
within it more or less complete coelomic space, and the portion of 
