202 PROCEEDINGS OF TEE NATIONAL MUSEUM. vol.39. 



As the nauplii develop these patches enlarge and deepen in color, 

 and as a result the external egg-cases, which up to the present stage 

 have been a uniform pale yellow, gradually show the color of the 

 patches through their walls and become more and more brown, until 

 this hue has become uniform. 



Internal structure of the nawplius. — Fig. 12 is a longitudinal hori- 

 zontal section through the body of the nauplius. It includes quite a 

 portion of the first antennfe {a'), simply the bases of the second pair 

 (a"), the rudiments of the upper lip (Ih), and mandibles (md), the 

 side walls of the body (w), and a section through the abdomen {ah). 

 Between the bases of the antennae may be seen the large frontal gland 

 in process of formation (fg). The entire center of the body is filled 

 with yoke globules, among which are scattered the large spherical 

 vacuoles. As can be seen the digestive system has not begun to 

 develop at this early stage, but first appears a little later. It was 

 impossible to distinguish any eye in the living nauplius and this 

 series of sections reveals no trace of it in the internal anatomy. 



There is an extremely rudimentary eye formed in the metanauplius, 

 as we shall see later, but it quickly disappears without leaving any 

 traces behind. 



Transformation of tJie nauplius into the metanauplius. — This first 

 portion of the stage which by courtesy we may call the nauplius is 

 gradually transformed into a real metanauplius. The transforma- 

 tion consists in a jointing of the first two pairs of appendages, the 

 appearance of the upper and under lips and all the mouth-parts, the 

 formation of two thoracic segments in front of the abdomen, and the 

 development on each of a pair of swimming legs. At first there is 

 only one of these free tliorax segments, while the abdomen is broad 

 and spatulate. But another segment is soon formed, and the abdo- 

 men is relatively narrowed. On each of these segments is developed 

 a pair of rudimentary legs, whose long plumose setse are at first 

 turned forward and inward along the sides of the body. As develop- 

 ment progresses the position of the setae changes, and they turn 

 gradually backward, until in the completed metanauplius they point 

 backward and inward (fig. 15). 



Body form of the metanauplius. — On freeing a metanauplius from 

 its surrounding envelopes in the same way as was done with the 

 nauplius, it presents the general appearance seen in fig. 16. The 

 body is very thick and stout; in fact, nearly spherical, as it was inside 

 the egg, and there have been radical changes in the appendages. 

 The first antennae (a') are now divided into three segments, of which 

 the terminal is much longer than the two basal, and are still each' 

 tipped with two plumose setae. The second antennae (a") have a 

 five-jointed exopod, each joint ending in a plumose seta, and a two- 

 jointed endopod, which retains the two terminal setae, but carries at 



