AMPHIOXUS LANCEOLATUS. 
385 
primary bar is a large space lined by an epithelium ( coel . ep.). 
This is the coelom, and is in free communication dorsally with 
the pharyngo-dorsal coelom, and ventrally with the coelom of 
the endostyle. The space becomes deeper and its walls longer 
as we ascend the primary bar, until it opens as a narrow but 
greatly extended space into the pharyngo-dorsal coelom. It is 
the raised-up walls of this space which form the pharyngo- 
pleural folds or coelomic pouches of the primary bars (see PI. 
XXXVI). 
In the tongue-bars there seems at first sight to be nothing 
which corresponds to the great coelomic channel of the primary 
bars. But when we trace the connection of the tongue-bars 
with the endostylar tract by means of transverse sections, we 
find that the canal within the rod which distinguishes the rod 
of the tongue-bar from the rod of the primary bar is in free 
communication with the endostylar coelom (figs. 6, 7, 8, PI. 
XXXVI B ). The canal within the hollow rod of the tongue- 
bar probably opens dorsally into the pharyngo-dorsal coelom, 
although the proof of this by means of transverse sections 
remains to be obtained by future inquiry. 
Variations in the amount and position of the chitinous 
deposit forming the rods of the pharyngeal bars are frequently 
found ; some of these are drawn in fig. 3, PI. XXXVI B. The 
most noticeable is the tendency to form a complete chitinous 
deposit embracing tbe supposed blood-vessels ( Bl. vess. of figs. 
1, 2, 3), and this may be either fused with the chitinous rod or 
detached from it as a separate piece. The bifid character of 
the rod of the primary bars is more obvious towards its extre- 
mities where it bifurcates (fig. 3, h ). 
A comparison of the structure of the gill bars of Amphioxus 
with the gill-filaments of the Lamellibranchs is instructive, 
and the latter may throw some light on the former. It can 
scarcely be maintained that the disposition of the blood-vessels 
in Amphioxus lends itself to the conclusion that we have here 
a highly efficient branchial respiratory apparatus. When the 
existence of extensive communications between the large 
coelomic spaces of Amphioxus and its blood-vessels are borne 
