320 



PROCEEDINGS OF THE NATIONAL MUSEUM, 



VOL. 39. 



Fig. 32. 



-Second antenna of nauplius of Ergasilus 



CENTB ARCHIDARtTM . 



and are the principal locomotor organs. They are of the usual form, 

 biramose, and armed with long plumose seta?. 



From the ventral surface of the basal joint of each there extends 

 inward a short masticatory process, endmg in a stout curved spme, 

 much longer than those found in ordinary free-swimming forms 



(fig. 32). The two spines 

 from the opposite sides curve 

 around the lower margin of 

 the labium and almost meet 

 at the midline. 



There is a third pair of 

 appendages corresponding to 

 the mandibles, but they are 

 much smaller than the others, are seldom used in locomotion, and are 

 usually carried folded back beneath the lateral margm of the body 

 so as to be invisible in a dorsal view. Furthermore when examined 

 they are found to be quite different in structure from the corre- 

 sponding pair in the Caligidse, and more like those of the free swim- 

 mers (fig. 33). They are biramose, 

 with the exopod considerably larger 

 than the endopod and made up of 

 three joints, each armed with a long 

 plumose seta. Thus these exopods, 

 while they are larger than those of 

 the free swimmers, are at the same 

 tune smaller than in the Caligidoe, 

 where they have the same number 

 of joints as the exopods of the second appendages. This diminu- 

 tion in the mandibles offsets the increase in size of the second anten- 

 nae and makes the general average about the same in both families. 



The endopods depart radically from the form seen in the Caligidas 

 and approach that of the free-swimmers. Each consists of a toler- 

 ably large and spherical proximal joint, the protopodite, to which 

 are attached two distal jomts entirely separate from each other, one 

 at the end of the proximal joint and the other on its ventral surface. 

 The one at the end is attached on a level with the dorsal surface 

 of the proximal joint. It is considerably widened along its distal 

 margin, which is armed with three spines and a curious flattened 

 lamina, shaped like the blade of a case loiife. The other joint forms 

 a masticatory blade which extends downward at right angles to the 

 axis of the basal joint. It is also widened at its distal end where it 

 carries two long setas of equal size. 



The median eye is placed far forward and is comparatively large; 

 it is almost entirely concealed in dorsal view by the supraesophageal 



Fig. 33. 



-Mandible of nauplius of Erga- 

 silus CENTRARCHIDARUM. 



