276 



PROCEEDINGS OF THE NATIONAL MU8EUM. 



VOL. 89. 



The lahrum is a chitin plate, usually somewhat obcordate or obo- 

 vate in shape, often wider than long, and situated between and 

 mostly posterior to, the bases of the second antennae. Its posterior 

 margin is evenly rounded and often has a large circular flap at either 

 end where it is joined to the ventral surface of the carapace. The 

 lip is well arched and covered in most species with short bristles or 

 hairs. In the Bomolochinse where the mouth-parts are close to the 

 second antennae the labrum is prominent and well defined, while in 

 the Ergasilinse where there is a considerable interval between the 

 second antennae and the mouth-parts, the labrum is fused with the 

 ventral sufrace of the carapace and often difficult to distinguish. 

 But in the early development stages of this latter family the upper 

 lip is prominent and occupies a position similar to that in the Bomo- 

 lochinae, and is also sparsely covered with hairs. (See fig. 35.) 



Fig. 10.— Mouth-pabts of Bomolochus kxilipes. an, Second antenna; la, labrum; lb, labium; md, 

 mandible; mx', first maxilla; mx", second maxilla; mxp, maxilliped. Between the termi- 

 nal JOINTS OF THE MANDIBLE AND SECOND MAXILLA CAN BE SEEN A HAIRY STRUCTURE SIMILAR TO 

 THE PAEAGNATHS IN FREE-SWIMMING FORMS. 



It must be therefore during the migration of the mouth-parts 

 backward away from the second antennae that the labrum becomes 

 fused with the ventral surface of the head. 



The labium is developed much later than the labrum, in fact, 

 after the other mouth-parts have been formed. It, too, is a chitin 

 plate attached to the ventral surface of the head. But it is flat- 

 tened and so thoroughly fused with that surface that it can not be 

 seen in the adults of many species. In some cases it is so narrowed as 

 to become virtually a post-oral bar similar to that described by 

 Claus and Hartog for Cyclops. To increase this similarity in the 

 genus Bomolochus a process runs forward and inward from either 

 end of the post-oral bar and below the shafts of the mandibles, thus 

 resembling the processes called paragnaths in Cyclops. These 

 processes are usually covered with short stiff hairs. 



