II ALIMENTARY CANAL AND HEART 29 
lips, running longitudinally forwards from the bases of the first 
maxillae, and often wrapping round the blades of the mandibles. 
It leads into a vertical oesophagus, which opens into a 
small globular stomach, lying entirely within the head; the 
terminal part of the oesophagus is slightly invaginated into the 
stomach, so that a valvular ring is formed at the junction of 
the two. The stomach opens widely behind into a straight 
intestine, which runs backwards to about the level of the telson, 
where it joins a short rectum, leading to the terminal or ventral 
anus. The stomach and intestine are lined by a_ columnar 
epithelium, and covered by a thin network of circularly arranged 
muscle-fibres; the rectum has a flatter epithelium, and radial 
muscles pass from it to the body-wall, so that it can be dilated. 
The only special digestive glands are two branched glandular 
tubes, situated entirely within the head, which open into the 
stomach by large ducts, one on each side. In Chirocephalus 
the gastric glands are fairly small and simple; in the Apodidae 
their branches are more complex and form a considerable mass, 
filling all that portion of the head which is not occupied by the 
nervous system and the muscles. Backwardly directed gastric 
glands, like those of the higher Crustacea, are not found in 
Branchiopods ; both forms occur together in the genus Webalia, 
but with this exception the forwardly directed glands are peculiar 
to Branchiopods. 
Heart.—In Branchipus and its alhes, and in Artemia, the 
heart extends from the first thoracic segment to the penultimate 
segment of the body, and is provided with eighteen pairs of 
lateral openings, one pair in every segment through which it 
passes except the last; it is widely open at its hinder end, and 
is prolonged in front for a short distance as a cephalic aorta, 
the rest of the blood-spaces being lacunar. 
In most, at least, of the other Branchiopods, the heart is 
closed behind and is shortened; in Apus and Lepidurus it only 
extends through the first eleven post-cephalic segments, while in 
the Limnadiidae it is shorter still, the heart of Zimnetis passing 
through four segments only. In all cases there is a pair of 
lateral openings in every segment traversed by the heart. 
~The blood of the Branchipodidae and Apodidae contains 
dissolved haemoglobin, the quantity present being so small as to 
give but a faint colour to the blood in Sranchipus, while 
