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 Cliiroceplmlu* 

 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 Nela/in, 

 but with this exception the forwardly directed glands are peculiar 

 to Branchiopods. 



Heart. In JSranchipus and its allies, 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 Lcpidurus it only 

 extends through the first eleven post-cephalic segments, while in 

 the Limnadiidae it is shorter still, the heart of Limnetis 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 Branchipus, while 



