200 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 39. 



but appear as rudimentary mandibles, without showing any traces of I 

 division into the usual two rami, of segmentation, or of the ordinary 

 nauphus armature of plumose setse. All the other appendages and 

 the rudiments of an upper and an under lip appear at the same time 

 with the third pair, and are developed directly into the forms found 

 in the metanauplius. It will be recalled that it was the appearance 

 of the two posterior pairs of mouth-parts side by side in Achtheres 

 percarum that induced Claus to first put forward the idea that they 

 developed as the exopod and endopod of the same appendage. Claus 

 himself corrected this idea in almost the last paper he published, 

 having found the two appendages to be distinct on the long-bodied 

 larvae of certain marine copepods. Moreover he discovered that the 

 posterior pair originated behind the groove separating the head from 

 the thorax, and were therefore thoracic, while the anterior pair were 

 cephalic. The same evidence can now be presented from the very 

 genus upon which Claus originally worked. If fig. 9 be examined 

 again it will be seen that the groove representing the boundary 

 between the head and thorax appears between the last two pairs of 

 mouth-parts, thus making the posterior pair thoracic and the anterior 

 pair cephalic. This groove only shows in sections and between the 

 nuclear centers or very beginnings of the appendages. It also dis- 

 appears before the appendages become visible externally, but it is 

 very distinct while it lasts and its significance can not be mistaken. 



The posterior body develops along with the appendages, and by 

 the time the first and second antennae are formed we find a thorax 

 and abdomen very similar to that which ordinarily appears in a 

 metanauplius larva. The balancers, which are so typical of nauplius 

 larvae, never appear at all ; in fact, the posterior body seems to start 

 in the metanauplius form from the very beginning and never displays 

 any of the nauplius characters. With these few words of explanation 

 as to the formation of the embryo we shall be the better prepared to 

 understand the remaining development inside of the egg. 



NAUPLIUS-METANAUPLIUS STAGE. 



In many of the parasitic as well as all of the free-swimming copepods 

 there is a long line of nauplius and metanauplius stages, distinctly 

 separated from one another by a throwing off of the old skin and 

 the formation of a new one. 



In the present species there is a complete fusion of all these stages, 

 with nothing left to indicate their presence except the formation of 

 a single nauplius cuticle. 



This larval integument, which is the fourth in chronological order, 

 is formed about the middle of the fused stage and may fairly be 

 taken to represent the close of the nauplius and the beginning of 

 the metanauplius stage. It is therefore the second molt of thai 



