288 ZOOLOGY 
crabs, and Gecarcinus amongst the Brachyura, have forsaken 
their natural element, and have come to live on land. They 
retain their gills, and the gill cavity contains both air and 
water. In Aivgus—a curious animal which is said to climb 
palm-trees at night—there is, however, a distinct modification 
of structure. The lower part of the gill space contains the 
numerous but small gills; this is shut off from the upper part 
by the infolded edge of the branchiostegite, and the cavity 
thus formed is a true lung, since it is full of air, and its 
walls are produced into numerous vascular folds, which receive 
impure blood from the body and return it oxygenated to the 
heart. Such a change, from water-breathing animals to 
terrestrial air-breathing animals, is paralleled in the Pul- 
monata, amongst which an intermediate form exists in the 
tropical water-snail Ampullaria, which has both a_ well- 
developed branchial cavity with gills and a well-developed 
pulmonary chamber, and uses them alternately to breathe 
water or air. 
The obscure question as to the nature of the body-cavity 
and the homology of the antennary or green gland in 
Crustacea has recently had some light thrown upon it by 
Weldon’s researches on Palaemon serratus and other Decapods. 
If the carapace of a Palaemon be removed a delicate sac will 
be found occupying the dorsal part of the cephalothorax, and 
extending from the anterior end of the head to the generative 
gland, to which it is closely attached. This cavity, termed 
the nephro-peritoneal sac, is lmed by epithelial cells, and 
exhibits many of the relations of a coelomic body-cavity. 
At its anterior end the sac gives off on each side a duct, 
which passes down and opens into the urinary bladder, thus 
putting the nephro-peritoneal sac in communication with the 
exterior through the excretory organ. The duct is lined by 
elandular cells, and gives off numerous caecal processes, which 
branch and ramify in the neighbouring tissues; in addition to 
these there is also a structure known as the “ end-sac,’ the 
cavity of which also opens into the urinary bladder. The 
walls of this end-sac are produced inwards into its lumen, 
dividing it up into a number of chambers lined with glandular 
cells; these walls are well supplied with blood-vessels, and 
