66(i PROCEEDINGS OF THE NATIONAL MUSEUM. 



In iiKujiilopx and i^flzoxtethU all four pairs of swiininini>; leo-,s arc 

 pn'soiit, and tliey have the .same nuiuher of joints and art> as fully 

 developed as in the adult. The basipods are 2-jointed, tlie proximal 

 joints decreasing- and the peripheral increasing in length from in front 

 backward. The exopods are 1-jointedand carry two long rowing sette 

 which are plumose and movable like tho.se on the antenna^ and mandib- 

 ular palps in the other species. The endopods of the first pair are 

 3-jointed. the first and second joints carrying sharp spines on their 

 posterior border and the third joint terminating in two similar spines 

 placed side hy side. The endopods of the three posterior pairs ar^ 

 2- jointed and carry but a single roAving seta. 



By referring again -to fig. 3 it will be seen that thes;^, four pairs of 

 fully developed swinmiing legs can be seen through the egg envelope 

 a day or two before the larvte emerges. 



When it does come forth, therefore, we find it using these same 

 appendages exclusively for locomotion, i. e., it hegins to sivlm <it once 

 in the mine manner and hy means of tlie same appendages whieh it is to 

 use all through life. 



We find in this respect a marked contrast between these two forms, 

 one {/negtdop.s) from the salt water, and the other {stizostethii) from 

 the fresh water, and the three other fresh-water species we are dis- 

 cussing. In the latter only the first pair of legs is at all developed, 

 the other three pairs being very rudimentary, mere stumps in fact, 

 immovable, and hence of no possible use in swimming. These larva', 

 therefore, are ol)liged to depend upon the second antenna? and the 

 temporar3Mnandil)ular palps for locomotion, being aided only slightly, 

 if at all, by the partiall}^ developed first pair of legs, i. e., these larvse 

 iegin to swim in an entirely different manner from the adult, and l>y 

 means of appendages temporarily developed for that purpose^ v>hich 

 aftenoards entirely disappear. 



Thus in their development these larvse show a partial metamorphosis, 

 and stand as a connecting link between the other parasitic copepods 

 (Eucopepods), with their complete metamorphosis through nauplius, 

 metanauplius, and the various cyclops stages, and species like vieg- 

 alojjs and stizostethii, in which there is really no metamorphosis at 

 all, at least no change in the number, position, segmentation, or 

 function of the various appendages, but only a few comparatively 

 trifling alteratioifs in size and form. 



Both the pointed, retractile proboscis and the mouth parts are 

 present in all the larvne. The latter take the form of a club-shaped 

 protuberance projecting from the mid line of the ventral surface 

 between the two pairs of maxillipeds. 



It is made up of an upper and an under lip and a pair of mandibles, 

 the maxillaj not l)eing yet developed. The chewing blades of the man- 

 dibles lie just beneath the upper lip, are sickle-shaped and curve in 

 toward each other from either side. 



