BRANCHIOPODA. 129 



winter from the cold. The heat of spring hatches them, and young 

 Daphniae are produced exactly similar to those which come from the 

 ordinary eggs. Schseffer affirms that they will remain for a long 

 period in a desiccated state without losing the vitality of the germ, 

 but none of those preserved in that condition by Jurine was ever 

 hatched. They are entirely free, or do not adhere to each other in 

 their peculiar cavities. In summer, according to Jurine, they may 

 be hatched in two or three days. In the climate of Paris, where 

 Straus observed them at all periods of the year, they require at 

 least one hundred hours. The foetus, twenty-four hours after the 

 production of the ovum, is a mere rounded and unformed mass, on 

 which, when closely examined, maybe seen obtuse rudiments of arms 

 in the form of very short and imperfect stumps glued to the body; 

 neither head nor eye is perceptible; and as yet, the green or reddish 

 body dotted with white, like the egg, exhibits no motion. It is only 

 at the ninetieth hour, and when the eye has appeared, and the arms 

 and valves are elongated, that the foetus begins to move. By the hun- 

 dredth hour it is very active, and finally, at the hundred and tenth 

 it only differs from the newly hatched animal in the setae of the oars 

 which are still glued to their stem, and in the tail of the valves 

 which is bent under and received between their inferior edges. To- 

 wards the end of the fifth day, the tail, which terminates the vulves 

 in the young animal, and the setae of the arms become free, and the 

 feet for the first time begin to move. The young being ready to 

 make their appearance, the mother lowers her abdomen and they dart 

 out. Newly laid eggs deposited in a glass jar, where they were 

 observed by Straus, were developed in this order. Jurine has also 

 furnished us with the result of his analogous observations upon the 

 successive changes in the embryo Daphniae, but made during the win- 

 ter, and as the eggs were not hatched till the tenth day, he could con- 

 sequently detect their development with more precision. The ovum, 

 on the first day, presents a central bubble, surrounded by smaller ones, 

 with coloured molecules in the intervals. These bubbles and mole- 

 cules appear destined to form the organs by approximating towards 

 the centre, and finally disappear. The form of the foetus begins to 

 be defined on the sixth day; on the seventh the head and feet are 

 distinguishable; on the eighth appears the eye as well as the intes- 

 tine; on the ninth the network of that eye begins to be visible, and 

 the bubbles have entirely disappeared, the central one excepted, 

 which contains the alimentary canal under the heart; on the tenth 

 the development of the foetus is terminated, the young Daphnia 

 issues from the matrix and for a moment remains motionless. 



The males, of those species at least observed by Straus, are very 

 distinct from the females. The head is proportionably shorter; the 

 Vol. III.— R 



