L EPTOC A RDIANS . 
1213 
digitiform (process-like) thickenings (tig. 365, vertically 
above /); and a constant current of water is thus kept 
up into the branchial sac (respiratory cavity, tig. 364, d ). 
The mouth cavity is delimited from this sac by a 
circlet of cirri (tig. 368), similar to those of the oral 
margin. 
The walls of the respiratory cavity, which also 
serves, however, as a pharynx or rather perhaps an 
(Esophagus, are laterally constructed of a network of 
cartilaginous rods (tig. 369), closely resembling the 
homologous structure in the Ascidians; but above and 
below the respiratory cavity has a continuous groove, 
the upper known as the epibranchial, the lower as the 
hypobranchial groove. The obliquely vertical rods set 
transversely in the body are rib-like growths on both 
sides, connected above with the outer sheath of the noto- 
chord (the so-called skeletogenous layer), the most pri- 
mitive rudiment of an axial skeleton. Below (towards 
the ventral side of the animal) these rods are free, some 
(the so-called primary bars) bifurcated, others (the so- 
called secondary bars or tongue-bars) single. The con- 
necting rods between them stand in the longitudinal 
direction of the body. Thus a network is formed, and 
this is faced everywhere, as the two grooves are also 
lined, with an epithelium of ciliated cells, exactly si- 
milar to those we have just seen in the mouth cavity. 
But the meshes of the net are minute, hardly visible 
even under a high magnifying power and under ordi- 
nary circumstances do not transmit anything but the 
clearest, purest water. In the adult state of the animal 
this water, which has now served its respiratory pur- 
pose, passes out on the sides of the respiratory cavity 
and into a. special chamber, a secondary compartment 
of the abdominal cavity. This chamber, which has been 
named by Ray Lankester" the atrium , surrounding 
the respiratory cavity on the sides and underneath and 
extending some way behind it, has a special aperture 
(the abdominal or atrial pore) behind on the ventral 
side (tig. 364, o) for the discharge of the water. Every- 
thing else that has accompanied the water into the re- 
spiratory cavity, and especially the food — Infusoria 
and other minute creatures — finds its way through the 
oesophagus proper, a tubiform prolongation of the epi- 
branchial groove, into a kind of stomach (fig. 364, just 
in front of i) with a long csecal appendage (/<), and all 
the useless matter now passes out through the straight 
intestine ( i ) and the anal aperture (e). The ctecal ap- 
pendage projects forward on the right side into the 
atrium, pressed close to the respiratory cavity. The 
whole digestive canal, the stomach and intestine, is lined 
with an epithelium of ciliated cells and exhibits a vibra- 
tile motion, just as the intestinal canal of the worms. 
The blood of Branchiostoma is colourless; and the 
circulation, of which little is yet known with certainty, 
may indeed be referred to the piscine type, for the vascular 
system is divided into certain regions which apparently 
lend themselves to a comparison with the heart and the 
great bloodvessels of fishes; but here too we are re- 
minded of the worms. Under the respiratory cavity 
runs a long contractile canal with a contractile vesicle 
to each ascending rod in the branchial network. This 
Fig. 369. A portion of the right side of the branchial basket in 
Branchiostoma lanceolatum, seen from without and powerfully magni- 
fied. After Vogt and Yung. 
a , secondary rods, so-called tongue-bars; h, primary rods. 
canal has been compared to the venous heart of fishes. 
Another contractile tube, anteriorly double (one on the 
right side, one on the left), posteriorly single, runs along 
the dorsal margin of the abdominal cavity, under the 
notochord. This canal has been compared to the aorta 
of fishes. Another similar canal, according to J. Mul- 
ler, connects on each side these two canals in front, 
and this has been compared to the so-called ductus ar- 
teriosus Botalli of the lower vertebrates and of the 
higher vertebrates in their embryonic stages. Along the 
intestinal canal run two ducts of a venous nature. The 
Quart. Journ. Micr. Sc., n. ser., No. LIX (July, 1875), p. 267; No. CXVI (Apr., 1889), p. 365. 
