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 _Nebalia_, but with this exception the forwardly-directed glands are peculiar to Branchiopods. =Heart.=—In _Branchipus_ 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 _Lepidurus_ 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 _Artemia_ has rather more, and the blood of _Apus_ is very red. The only other Crustacea in which the blood contains haemoglobin are the Copepods of the genus _Lernanthropus_,[19] so that the appearance of this substance is as irregular and inexplicable in Crustacea as in Chaetopods and Molluscs.