STATE GEOLOGIST. 53 



the median dorsal line and extends toward the abdomen. It 

 grows much more rapidly than the other parts of the shell and, in 

 a later stage, forms a comparatively enormous tail, which curves 

 under the animal between the shell valves which now extend be- 

 yond the body. This "tail" extends well along the ventral 'mar- 

 gin of the shell and reminds, by its position, of the tail of a fright- 

 ened dog. The true tail, or post-abdomen, is, in the meanwhile, 

 well developed and is constantly kicking the useless protuberence 

 of the shell upwards. As the animal leaves the egg this projec- 

 tion becomes straightened as in the young D. pulex, finally be- 

 coming the still considerable spine, though it is proportionately 

 much shorter than in the embryo. The spine becomes shorter 

 with successive moults and the mature form has only a slight 

 rounded knob in place of a spine more than half the length of 

 the body. 



The use of the long spine in the young Daphnia is a matter of 

 interest. Its length agrees pretty well with that of the brood 

 <iavity and it seems possible that it serves to prevent the shell 

 from bending abruptly down when it is only partially removed 

 during the moult and thus breaking off and so leaving a portion 

 of the clothing of the brood-cavity therein to become a source of 

 irritation. This is more necessary for the young since the brood 

 ■cavity is narrow and the shell weak, so that while the outer shell 

 is removed like a glove from the finger, it can not be pulled up- 

 ward or downward, but directly backwards. It is well known that 

 male Daphnise often have the spine, while the females may have 

 none, and here again it is possible that the narrower cavity over 

 the abdomen requires this assistance, while this is not the case 

 with the females. 



The shell gland is early formed and the branchial lamellae of the 

 feet appear almost simultaneously with the feet themselves as 

 distinct lobes. The branchial chamber is not a simple chamber, but 

 is essentially a curved tube as can be very well seen in the last foot 

 of the adult. This tube doubles upon itself and crosses in the 

 manner of a loop and a constant stream flows rapidly through it. 



The nervous system is, at first, paired from beginning to end 

 and first unites anteriorly, the ocular ganglia fuse after the union 

 of the two pigment flecks in the compound eye, then the cephalic 

 ganglion is formed by the union of the two preoesophagal ganglia, 

 the commissures passing about the oesophagus. I have not been 

 able to determine if the suboesophagal ganglia become fused. 

 From the anterior ganglia spring the nerves to the antennae and 



