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 Birgus — 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 AmpuUaria, which has both a weU- 

 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 lined 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 

 glandular 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 



