496 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



Thi« cone is often partially or even almost completely concealed by 

 the border of the frontal plate in a dorsal view, so that it is only by 

 turning - the animal over on its back that one could be sure whether it 

 has lunules or not. This is the condition with the entire genera Cali- 

 godes, Synestius, and Parwpetalms, and rarety in some species of Caligus 

 like C. diaphamis. 



The carapace, like all the rest of the body, is covered on both the 

 dorsal and ventral surfaces by a thin cuticle. At the margin where 

 these two cuticles come together they are fused and form a wide, per- 

 fectly transparent border along the frontal and lateral edges. Being 

 smooth and flexible this border can be applied very closely to the sup- 

 porting surface and forms thereby a tight joint which greatly aids in 

 prehension, as already noted (p. 486). 



The eyes are situated on the median line, about one-third the dis- 

 tance from the front of the carapace. They are two in number, so 

 closely approximated as to be partially flattened, and are embedded 

 in a mass of pigment which lies wholly beneath the carapace. Each 

 consists of a spherical mass of pigment flattened on the inner side, 

 where it is separated from its fellow by a thin layer of chitin, lined 

 with the same pigment. 



The lens is spherical and projects about half its diameter from the 

 outer or anterior margin. Behind the lens is a retina made up of a 

 single row of relatively large cells, which are lined on the inner side 

 with a layer of pigment. This pigment is usually black or very dark 

 wine-red in color, while the lens is "colorless and perfectly transparent. 



In quite a number of species, scattered through all the different 

 genera, the eyes are invisible (in preserved specimens) to even the 

 most careful scrutiny. But it seems probable that they are merely 

 concealed by overlying pigment and not really lacking. This point 

 can be determined only by a study of sections which are not at pres- 

 ent available. 



From a study of the early development we find that these eyes are 

 original^ placed much farther back in the carapace and are separated 

 by a greater distance from each other, and that they afterward 

 migrate forward and inward toward the mid line, until they are so 

 thoroughly fused as to appear as one eye with two lenses. 



The free thoracic segment is small in nearly all the species; it rep- 

 resents the fourth thoracic segment of free-swimming copepods and 

 carries the fourth pair of swimming legs attached to its outer mar- 

 gins (F. S. fig. 2). In all the species figured, with one or two doubt- 

 ful exceptions ( C. dubius and C. fallax), it is so much narrower than 

 the carapace and genital segment, especially where it joins the former, 

 as to appear like a wasp waist connecting the two. This appearance 

 is heightened in Oaligodes by a considerable lengthening of the seg- 

 ment. The rare instances in which it is figured as double instead of 



